In 1271, Kublai Khan established the name of Dayuan, officially became emperor, and began to implement plans to attack the Southern Song Dynasty from the south. At this time, the Southern Song Dynasty occupied Lin'an and other places, and Kublai Khan desperately needed a passage to transport grain from the south to the north. The Mongols, as nomadic people on horseback, have always been accustomed to land combat, and when they marched south, they also cleverly knew how to use the sea, they not only took in the betrayed Southern Song navy, but also had a large marine warship and a large marine cargo ship; Overseas trade from Southeast Asia to the Persian Gulf was also established from the use of piracy, as well as a stable maritime food transportation system.
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There are several islands at the mouth of the Yangtze River, the largest of which is Chongming Island. Surrounded by water, Chongming Island is easy to defend and difficult to attack, and in the 13th century, it was no longer controlled by the Song Dynasty government, and became a free paradise for pirates. In the middle of the 13th century, there were two famous pirates here: Zhu Qing and Zhang Xuan, who led the crowd to "scream and plunder", sold salt, plundered rich merchants and giants, and at most gathered nearly a thousand pirates, 500 ships, and the scope of activity extended from tonghai in the south to Jiaodong Peninsula and Laizhou Bay in the north, and was later recruited by the Southern Song Dynasty.
In 1273, Zhu Qing and Zhang Xuan betrayed the Southern Song Dynasty and became an important force in the Yuan Shui Army, and Zhu Qing was awarded the military position of acting as the deputy of Guan Jun Qianhu. Three years later, the two pirates received a mission: to transport the Southern Song Dynasty treasury. In that year, the Yuan army captured Lin'an, and the Southern Song dynasty ministers suggested that Xiang Boyan transport the southern Song Dynasty's treasury, including archives, charts, and sacrificial utensils, to the capital of the Yuan Dynasty at that time, Yuan Dadu, for the revision of the History of the Song Dynasty. However, at that time, the Huaidong region was still under the control of the Southern Song generals, so Boyan thought of sea transport, and he gave this task to Zhu Qing and Zhang Xuan, and succeeded.
Six years later, the grain transported to Beijing through the Grand Canal was delayed, and the freight of the inland grain transport was expensive, so Bo Yan remembered the transfer of the Song Chamber's treasury by sea route in 1276. After Bo Yan reported this law to Kublai Khan, Kublai Khan weighed the pros and cons of different shipping schemes, decided to adopt the suggestion of Xiang Boyan, ordered the construction of 60 sea ships, recruited a large number of shipwrights and crew members who were familiar with the sea situation, and appointed Zhu Qing and Zhang Xuan as the sea route grain transport households, responsible for sea transport.
In 1282, the construction of 60 flat-bottomed boats of the Yuan Dynasty government was completed, and under the auspices of Zhu Qing and Zhang Xuan, the boats loaded grain in the Liuhe River in today's Taicang County, Jiangsu Province, with large ships loading 1,000 stones, small boats loading 300 stones, and 60 flat-bottomed sea boats carrying a total of 46,000 stones of grain. After leaving the port, the fleet passed through Yangzhou, crossed Huanglian Shazui and Wanli Long Beach east of Haimen County, then sailed northwest, reached Huai'an and Yancheng Counties, and then headed north, along the long coastline all the way north, turning west in today's Rongcheng area of Shandong, reaching Dengzhou Ancient Port, and continuing westward into today's Haihe estuary, the whole voyage lasted more than four months, and all the grain arrived in Beijing in March 1283. Although the numbers were modest, they made a good start — a voyage that proved the feasibility of transporting food by sea. At that time, Zhang Huangwei had a poem "At the beginning of the country, the sea was shipped from Zhu Zhang, and a million building ships crossed the ocean", which described the scene of grain transportation at sea at that time.
The success of the voyage made the rulers of the Yuan Dynasty pay more attention to maritime transportation, and by the 20th year of the Yuan Dynasty (1283), the imperial court built 2,000 boats, and by the 22nd year of the Yuan Dynasty (1285), another 3,000 ships were added, all of which were put into grain transportation. The amount of grain transported by sea increased year by year, and by the 27th year of the Yuan Dynasty (1290), the annual volume of grain transport reached 1.59 million stones. Kublai Khan was bent on invading Japan and Champa (present-day southern Vietnam), requiring a large number of ships and sailors, and the economic power of the Yuan Dynasty at that time was unable to build flat-bottomed boats suitable for transporting large quantities of cargo. So he decided to rely on merchants and pirates to complete this dangerous sea voyage.
He entrusted Zhu Qing and Zhang Xuan with the task of organizing personnel to transport grain. The two had the power to choose their personnel, and could even confer official positions without the approval of the imperial court. In the early 1680s, the cost of building a large ship with a displacement of 1,000 materials (the load unit of ancient ship survey ships) was about 100 ingots, while the freight of 1,000 quintals was 170 ingots, so the profit was so high that the sea merchants certainly did not miss the opportunity. Therefore, Zhu Qing and Zhang Xuan's men immediately gathered salt merchants, salt workers, ship owners, fishermen, shipping officials and sailors tens of thousands, and the two also invited pirates from the southeast coast to join.
This policy gave merchants huge profit margins, encouraged them to participate in many ways, and finally achieved unprecedented success in grain transportation. For more than half a century, the sea route was the main route of grain supply in the Yuan Capital, reaching its peak in 1341, transporting 3.6 million stones of rice, unprecedented in Chinese history.