Sima Dan is named Peng Zu, the son of Emperor Kang, and his biological mother Chu Suanzi. Emperor Kang died of illness in September 344, and Sima Dan, who was two years old (in vain), was established as emperor and changed to Yuan Yonghe. Empress Dowager Chu Suanzirule as regent, He Chong, Prince of Huiji Sima Yu and others assisted in the administration. Emperor Mu reigned for seventeen years, most of which was ruled by the Queen Mother and powerful ministers. In 357, Emperor Mu took over the throne, but the real power was still not in his hands. He died of illness in 361, at the age of nineteen, without any heirs.
The Emperor Mu Dynasty was a period when the Eastern Jin country was relatively strong: Huan Wen, the governor of Jingzhou, conquered the Chenghan in the west and recovered Bashu in 347; the Hou Zhao in the north collapsed, Ran Min killed Hu, the former Yan rose, and the former Qin was established. The Eastern Jin court successively led the northern expedition with Yin Hao, Huan Wen and others. Huan Wen's prestige was growing through the destruction of Shu and the Northern Expedition, and he was constraining each other with Sima Yu and Yin Hao in the court. Eastern Jin's clan politics entered a new stage, and the Xie family, Huan family, etc. also gradually emerged.
Culturally, in the ninth year of Yonghe (353), Wang Xizhi held a ritual ceremony at the Orchid Pavilion in Shanyin, Kuaiji, and wrote the "Preface to the Orchid Pavilion"; famous monks such as Zhi Dun socialized with scholar-bureaucrats, metaphysics and Buddhism stirred up each other, and Jiangnan culture entered a prosperous stage. Emperor Mu died of illness in May 361.
Emperor Mu himself came to the throne at a young age and never held real power throughout his life. The historical merits and demerits were mainly borne by the Queen Mother and powerful ministers. However, during his reign, the Eastern Jin destroyed the Chenghan, regained Luoyang, and Huan Wen's prestige rose. Later Zhao in the north collapsed and the former Qin rose, and the political landscape underwent important changes. Culturally, the Lanting Elegant Collection merged with Xuanfo, making the Yonghe period one of the pinnacles of Eastern Jin culture, and had a profound impact on the history of Chinese calligraphy and thought.