Instead, He was forced to advanced northwest and then west to face Former Qin, ruled by the ethnically-Di emperor Fu Sheng.

Fu Jiān made him the governor of Longdong Commandery (隴東, roughly modern Baoji, Shaanxi).

In 371, he was part of the campaign against Chouchi's ruler Yang Cuan (楊篡), and in 373, after Former Qin seized the modern Sichuan, Chongqing, and southern Shaanxi from Jin, Yao Chang was made the governor of Ning Province (寧州, modern southern Sichuan).

In 383, Fu Jiān prepared a major attack on Jin, intending to destroy it and unite China.

Yao Chang was put in charge of the armies of the southwest (on a separate front away from the main front), and as Fu Jiān was handing out the command assignments, he gave Yao Chang the title General Longxiang (龍驤將軍) -- a title that he himself carried previously when he overthrew Fu Sheng—and he commented, apparently in a jocular tone: Another general, Dou Chong, immediately objected, stating that it is a bad omen for the emperor to joke, and Fu Jiān grew briefly silent.

In 385, an incident involving the city of Xinping (新平, in modern Xianyang, Shaanxi) helped to further establish Yao Chang's reputation as wily and treacherous.

Gou believed him, but as soon as he exited the city with the 5,000 remaining people, Yao Chang surrounded them with his troops and slaughtered them all, with only a single person who escaped.

In fall 385, Fu Jiān abandoned Chang'an, long under siege by Western Yan, and he sought to try to find food supplies.

When he arrived at Wujiang Mountain (五將山, in modern Baoji, Shaanxi), Yao Chang sent his general Wu Zhong (吳忠) to surround Fu Jiān, eventually capturing him and delivering him to Xinping to be imprisoned there.

He created his wife Lady She as empress and his son Yao Xing as crown prince.

However, Yao Chang was able to gradually subdue the other Former Qin generals of the region, taking advantage of Fu Deng's cautiousness.

He also, at the same time, seized the remaining cities held by Western Yan west of the Yellow River.

In 389, after losing several battles to Former Qin, Yao Chang desecrated Fu Jiān's tomb and whipped his body before reburying it.

It is recorded in the Book of Jin that he was haunted by Fu Jiān's spirit in his dreams and became insane.