Around 616, Zhai Rang gathered a group of men to resist the rule of Emperor Yang. Xu Shiji joined Zhai, and suggested to him that instead of pillaging the people of the region, which Zhai had done to maintain himself and his own men, it was inappropriate to be robbing people of their homeland. Rather, he suggested that, as there were plenty of travellers on the Grand Canal, that they should rob travellers instead. Zhai agreed, and their pillaging raids instead targeted travellers and official messengers. Many people joined Zhai, and when the key Sui general Zhang Xutuo (), who had defeated many agrarian rebels, attacked, Xu killed Zhang in battle late in 616, allowing Zhai to become even more acclaimed. Around this time, he also became close friends with another general under Zhai, Shan Xiongxin (), swearing to be brothers and to die on the same day.