He distinguished himself under the commands of Gao and Wang Zhengjian (王正見), particularly in a battle against Greater Bolü (大勃律, near modern Gilgit-Baltistan, Pakistan) in 753, eventually commissioned as a military governor (jiedushi).
After failing to defeat An Lushan's rebellion in 755, however, both he and Gao were executed due to accusations against them made by the eunuch Bian Lingcheng (邊令誠).
His maternal grandfather had committed a crime and was exiled to Shymkent,[citation needed] to serve in the Tang dynasty army, as part of the army for Anxi Circuit (安西, headquartered in modern Aksu Prefecture, Xinjiang) with the responsibility to guard the south gate of the non-Han portion of the city.
Feng thereafter became known to the Anxi headquarter staff, and was given the honorific title of castle defender and made Gao's assistant.
In 747, Feng served under Gao in the conquest of Lesser Bolü (小勃律, a city state centering on modern Gilgit, Pakistan).
(Subsequently, though, due to machinations of An Sishun, then military governor of Hexi, who wanted to remain there, Gao was instead recalled to the capital to serve as a general of the imperial guards.)
When Wang Zhengjian was named as the military governor of Anxi to replace Gao, he asked Feng to serve as his deputy.
In 755, An Lushan, the military governor of Fanyang Circuit (范陽, headquartered in modern Beijing), launched a rebellion against Emperor Xuanzong's rule.
Feng, initially believing that An's rebellion could be defeated quickly, volunteered to head to the eastern capital Luoyang to defend it and then to counterattack.
Emperor Xuanzong agreed, and commissioned Feng as the military governor of Fanyang and Pinglu (平盧, headquartered in modern Chaoyang, Liaoning) to replace An.
Once he got there, he gathered 60,000 men and destroyed the Yellow River bridge to prepare for attack, but quickly found out that he had a major problem—there were insufficient weapons available for his troops.
Meanwhile, Gao Xianzhi, who was the acting supreme commander of the forces, was stationed at Shan Commandery (陝郡, roughly modern Sanmenxia, Henan).
Feng retreated to Shan, where Emperor Xuanzong sent an order stripping him of titles but allowing him to remain in the army under Gao.
Feng warned Gao that Shan Prefecture was difficult to defend—that they should retreat to Tong Pass and fortify its defenses against An's attack.
Meanwhile, though, Feng was detecting signs that he might be in trouble, and anticipatorily wrote a final submission to Emperor Xuanzong, warning him about An's strengths and cautioning against overconfidence, as the other officials at the time generally believed that An could be defeated quickly.