In the 19th year of Zhenguan (645), Xue abandoned agriculture to join the army and followed Emperor Taizong on his first expedition to Korea. In June of that year, Tang Jun marched into Anshi Town, an important town in Koguryo, and Koguryo Mo left the branch and Gai Suwen intercepted it with an army of1.5000. Emperor Taizong laid an ambush in the valley and sent a small cavalry to lure the enemy deeper. After Koguryo's troops entered the valley, they fought against Tang Jun by numbers. According to the Book of Old Tang Dynasty, when the North Korean army was in chaos, Xue dressed in a white robe and armed with a halberd rode out to kill the enemy and beheaded several generals while the North Korean army was in chaos, which led to the chaos of the North Korean army. Because Xue was particularly conspicuous in his white robe (he deliberately did this to attract the attention of Emperor Taizong), he asked the general's name and answered Xue. In this battle, Tang Jun defeated Koguryo Army and beheaded more than 20,000 people. After being occupied in Anshicheng for a long time, Tang Jun returned to Yingzhou, and Emperor Taizong met Xue alone, praising his bravery and hoping that he could serve the imperial court. Later, Xue was appointed as a general to guard Xuanwu Gate. Xue was grateful to Emperor Taizong for his kindness and vowed to serve the Tang Dynasty to the end.