He was mostly active in Qin's conquest of western China, helping Fu Jian in conquering Chouchi, Yizhou and Liangzhou.
Although not as grand as his contemporaries, Deng Qiang and Zhang Ci, the Spring and Autumn Annals of the Sixteen Kingdoms classed him as an important general of Qin whose merits were comparable to the two.
Yang An led the vanguard together with Mao Sheng (毛盛) while Fu Jian personally campaigned against Cao Gu.
[6] In 366, Yang An, Wang Meng and Yao Chang attacked Nanxiang Commandery (南鄉) in Jingzhou, and Jin in response sent Huan Huo to push them back.
As Huan Huo reached Xinye, the three generals withdrew, claiming more than ten thousand households north of the Han River.
After asking for help, Fu Jian sent Yang An to reinforce Li at Fuhan (枹罕縣; in present-day Linxia County, Gansu).
The situation was resolved after Wang Meng defeated Zhang Tianxi's army near Fuhan and convinced him to withdraw his troops.
When Wang Meng arrived to help after capturing Huguan, the two devised a plan to have Zhang Ci infiltrate the city.
After Zhang Ci did so, Wang Meng and Yang An's forces flooded the city and captured its defender Murong Zhuang.
They were intercepted by the Qin general Zhu Yong (朱肜), so Zhou Xiao, not wanting to risk their lives, quickly surrendered to Yang An.
[13] The following year, a rebellion arouse in Shu, headed by Zhang Yu (張育) and Yang Guang (楊光).
Zhang Yu's regime was supported by the Jin, who helped him defeat Yao Chang, but although he lasted long enough to declare himself King of Shu, his followers started turning on each other and the rebels were disorderly.