In fall 621, Emperor Gaozu commissioned Li Xiaogong, with Li Jing as his assistant, to launch a major attack on Xiao Xian's Liang state, with Li Yuàn and the other generals Tian Shikang (田世康) and Zhou Faming (周法明) attacking on other fronts. At that time, the Yangtze River had a high water level and was flowing rapidly. While many officers under him suggested delaying the campaign, Li Jing suggested to Li Xiaogong that the rapid water was a good opportunity to launch a surprise attack on Xiao Xian's capital Jiangling (江陵, in modern Jingzhou, Hubei). After defeating the Liang general Wen Shihong (文士弘), Li Xiaogong put Jiangling under siege, cutting off Xiao Xian's communications with the armies in the rest of his territory. Li Jing suggested floating the Liang ships that they captured down the Yangtze, to confuse the approaching Liang relief forces into believing that Jiangling had fallen already. Xiao, not knowing that the relief forces were only a few days away, surrendered to Li Xiaogong. By the suggestion of Li Jing and Xiao's official Cen Wenben, who had persuaded Xiao to surrender, Li Xiaogong strictly prohibited pillaging and retribution against Liang generals. The Liang provinces, upon hearing the news of Jiangling's fall, soon largely submitted to Tang. Li Xiaogong delivered Xiao to Chang'an, where Emperor Gaozu executed him. For his accomplishments, Li Jing was created the Duke of Yongkang. Subsequently, Emperor Gaozu commissioned Li Jing to head south to persuade the modern Guangdong and Guangxi region, which had submitted to Xiao Xian and another rebel ruler, Lin Shihong the Emperor of Chu, to submit to Tang. Li Jing advanced to Gui Prefecture (桂州, roughly modern Guilin, Guangxi) and was able to persuade several of the most major warlords of the region, including Feng Ang (馮盎), Li Guangdu (李光度), and Ning Zhenzhang (甯真長) to submit to Tang. Subsequently, Li Jing was made the commandant at Gui Prefecture.