In 645, Li Daozong accompanied Emperor Taizong in attacking Goguryeo, and he served as assistant to the forward commander Li Shiji. In summer 645, they captured Gaemo (, in modern Fushun, Liaoning), and then defeated Goguryeo forces at Yodong/Liaodong (, in modern Liaoyang, Liaoning), allowing Emperor Taizong to then capture Liaodong. However, after another victory over Goguryeo forces at Ansi (, in modern Anshan, Liaoning), Emperor Taizong became bogged down in his siege of Ansi, and Li Daozong suggested, instead, that he skip Ansi and directly attack the Goguryeo capital Pyongyang. Emperor Taizong initially agreed, but Li Shiji and Zhangsun Wuji opposed, believing that if they did not capture Ansi first, Ansi's defender (a capable general known in Korean popular legends as Yang Manchun, although whether that was his real name is unknown) would attack them from the rear. Emperor Taizong therefore continued to siege Ansi. During Ansi's siege, part of Ansi's city walls collapsed, and Tang forces had an opportunity to enter, but at that time, Fu Fu'ai (), an officer under Li Daozong, was not paying attention, and instead Goguryeo forces fought out from the breach and captured the part of the offensive earthworks that Li Daozong was responsible for, using it instead for defense. For this offense, Emperor Taizong executed Fu, and when Li Daozong, baring his feet to show remorse, requested forgiveness, Emperor Taizong rebuked him but kept him in command of his forces. Subsequently, with Ansi's defenses holding, Emperor Taizong withdrew, with Li Shiji and Li Daozong serving as the rear guard. After returning to Chang'an, Li Daozong offered to resign on account of illness, and Emperor Taizong made him the director of imperial clan affairs, a much less stressful position.