After Emperor Taizong took the throne, he made Li Shiji the commandant at Bing Prefecture (, roughly modern Taiyuan, Shanxi). In 629, when Emperor Taizong launched a major attack against Eastern Tujue's Jiali Khan Ashina Duobi (a.k.a. Jieli Khan or Illig Qaghan), commanded by Li Jing, Li Shiji was one of the major generals under Li Jing's command and was in charge of one of the four prongs of attack. In spring 630, after Li Shiji defeated Eastern Tujue forces at Baidao (, in modern Hohhot, Inner Mongolia) and after Li Jing defeated Ashina Duobi's main forces and forced Ashina Duobi to flee, he met with Li Jing and decided to, despite the fact that Ashina Duobi claimed to then seek peace, attack Ashina Duobi, defeating him again and capturing most of Ashina Duobi's remaining forces. (Ashina Duobi himself fled further and was later captured by forces under the command of Emperor Taizong's cousin Li Daozong the Prince of Rencheng County.)