During his final years, he demoted himself from Emperor and used the unique title of "Commoner Heavenly King" (庶人天王).
Murong Sheng was described as a talented strategist and general, but he overreacted to what he perceived to be his father's weaknesses—being overly lenient and weak—and was harsh in his reign, causing his officials to be constantly insecure and looking to rebel against him.
In the Record of Jin, Murong Sheng was described as a strong and courageous man who excelled in archery skills.
His judgment was correct, for although Murong Chong captured Chang'an in summer 385, he did not govern his people well, and his Xianbei people became resentful that he stayed in Chang'an (both because he liked the city and because he feared Murong Chui) rather than seeking to head back east to their homeland.
It took several months for them to reach the Later Yan capital Zhongshan (中山, in modern Baoding, Hebei).
In spring 396, Murong Chui's campaign against Northern Wei was initially successful, but he then got ill and was forced to withdraw back to Zhongshan, and he died on the way.
Murong Sheng, not trusting his father-in-law, advised his father to secretly advance south to try to join forces with his granduncle Murong De, who had been still defending Yecheng, not realizing that by that point Murong De had abandoned Yecheng and regrouped at Huatai (滑台, in modern Anyang, Henan) and declared his own independent state of Southern Yan.
Initially, the people were glad that Murong Sheng was victorious, believing that he would be a capable ruler.
Murong Sheng's reign, indeed, was one in which he showed strong personal abilities, and yet at the same time alienated the officials and the people with his harshness.
These included: However, Murong Sheng was also praised by historians for his attention to ordinary criminal cases, and that he was able to institute a system where he personally heard criminal appeals and was able to discern the truth without resorting to torture (a common interrogation device in Chinese history).
Through the next few years, there would be battles with Northern Wei, with neither side being able to decisively gain an advantage.
He also soon, to show humility, stopped using the title emperor, instead referring to himself as "Commoner Heavenly King."
Murong Sheng personally led the palace guards to fight with the rebels and was initially successful.
Murong Sheng, despite the injury, ordered the guards to clear the palace and go on the correct alert posts, before dying.
After his death, his mother Empress Dowager Ding, who had carried on an affair with his uncle Murong Xi, would bypass Crown Prince Ding and make Murong Xi emperor to succeed him.