More than 20,000 people were sent to pacify, and Zhai Rang was frightened and tried to escape, but Li Ji stopped him. The Wagang army under his command used military tactics and annihilated all the Sui army. At that time, Li Mi, the Duke of Pushan, participated in Yang Xuangan's rebellion and fled in defeat. Li Ji and Wang Bodang knew that Li Mi was a hero in the world, and together they persuaded Zhai to let Li Mi be the main one. In 617, Li Mi proclaimed himself the Duke of Wei, and Li Ji was made a great general of the Right Wu Hou and commanded the army.
When the Sui Dynasty learned the news, he led an army to attack Li Mi. As a result, Li Ji used a strange trick to defeat Wang Shichong on both sides of the Luoshui River, and Li Mi also made him the Duke of Donghai County. At that time, the Natural and Man-made Disasters of the Sui Dynasty caused starving people everywhere, and the Sui Dynasty did not take this matter seriously, and tens of thousands of people starved to death every day. Li Ji asked Li Mi for support from the victims, and Li Mi agreed and crossed the Yellow River from Yuanwu with 5,000 men to attack the Sui Dynasty defenders in Liyangcang. After the breakthrough, the warehouse was opened to invite the people to receive food at will, so that the team continued to grow. Later, conflicts broke out within the Wagang army, and Zhai Rang was killed by Li Mi. In the chaos, Li Ji was also slashed by the soldiers and seriously injured.
In March 618, the Sui Emperor was killed in Jiangdu. In May, Li Mi sent Li Ji to recruit the rebel general Wang Deren in revenge for killing Fang Yanzao. Li Ji defeated Wang Deren and forced him to surrender to Li Yuan, the Tang king. Xu Shixun defended Liyangcang, Yu Wenhua and led his army to attack the city on all sides, and the situation was critical. Li Ji dug tunnels from the city and suddenly appeared outside the city, defeating Yu Culture and forcing him to break the siege. In this year, The Prince of Taifu, YuanwenDu, and others proclaimed himself emperor of Luoyang by Yang Tong, the Prince of Yue. Emissaries were sent to pardon Li Mi and others, and Li Ji was re-awarded the title of Grand General of the Marquis of Right Wu.
In October 618, Li Mi was defeated by Wang Shichong and submitted to the Tang Dynasty. All the territories that Li Mi had originally commanded were taken over by Li Ji. In 619, Li Ji sent emissaries to Li Mi to write to li mi to return what originally belonged to li mi. When the emissary first arrived at the imperial court, Li Yuan heard that Li Ji did not play the table, but only wrote a letter to Li Mi, which was quite strange. After the emissaries informed Li Yuan of Li Yuan's intentions, Li Yuan was so moved that he was made the Duke of Lai. Since during the Sui Dynasty, Li Ji's original name was always called Xu Shixuan, so after returning to the Tang Dynasty, he was given the surname Li, and then given the first high-class house. At this point, Xu Ji changed his name from Xu Shixun to Li Shixun.
In September 619, the Hebei warlord Dou Jiande attacked Xiangzhou, and Li Shentong, the king of Huaiyang, led an army to flee to Liyang. Dou Jiande captured Liyang in November, and Li Shentong, Wei Zheng, and Princess Tong'an were all captured. Li Ji had to return to surrender because Li Gai was captured. Li Ji wanted to submit to the Tang Dynasty, but feared that the move would affect his father. After privately consulting with Guo Xiaoke, he decided to make meritorious contributions to Dou Jiande first, gain trust, and then make plans. Soon after, Li Ji defeated Wang Shichong's army in an attack on Yujia, and Li Ji gained Dou Jiande's trust. Later, dou jiande was persuaded to go to Henan to wait for an opportunity to kill him, but it was delayed due to dou jiande's wife giving birth. Li Ji was later unable to kill Dou Jiande, and the plan was leaked. Therefore, he could only lead dozens of troops with Guo Xiaoke to surrender to the Tang Dynasty.