In 369, when the Jin general Huan Wen launched a major attack on Former Yan, seeking to destroy it, Huan was initially successful and reached Yecheng's vicinity. Murong De's older brother Murong Chui volunteered to make one last attempt against Huan, and Murong De was a general under his command. Together, they dealt Huan a major defeat, forcing Huan to abandon his campaign. However, later that year, Murong Chui, under suspicion by Murong Ping and Murong Wei's mother Empress Dowager Kezuhun, was forced to flee to Former Qin, and because of the close relationship Murong De had with Murong Chui, Murong De was removed from his post as the mayor of Yecheng. After Former Qin conquered Former Yan in 370, Murong De, as other princes of the Murong clan did, surrendered to Former Qin forces.