Emperor Jin Chengdi 晉成帝 (r. 325-342), personal name Sima Yan 司馬衍, courtesy name Shigen 世根, was a ruler of the Eastern Jin dynasty 東晉 (317-420). He was the oldest son of Emperor Ming 晉明帝(r. 322-325) and Empress Yu Wenjun 庾文君 (297-328). He was appointed Heir Apparent (huang taizi 皇太子) with the age of 5 sui and succeeded his father to the throne just one year later. Regency was therefore nominally taken over by the Empress Dowager, his mother, while the government was managed by Wang Dao 王導 (276-339), Minister of Works (sikong 司空) and Yu Liang 庾亮 (289-340), the Director of the Imperial Secretariat (zhongshu ling 中書令) and older brother of the Empress Dowager.
Yu Liang was able push away Wang Dao from the centre of government and began to reign in an autocratic manner, not shying away from killing his opponents. In 327, the chamberlain (neishi 內史) of the princedom of Liyang 歷陽 (today's Hexian 和縣, Anhui), Su Jun 蘇峻 (d. 328), rose in rebellion and marched against the capital Jiankang 建康 (Nanjing 南京, Jiangsu). A year later he took Jiankang and allowed his troops to loot, rape and plunder. Su Jun took the emperor as a captive and detained him in Shitoucheng 石頭城, a suburb of the imperial capital (today called Qingliangshan 清涼山). A united force of local commanders finally overcame the rebel and Su Jun died.
Yu Liang was given the command over the troops in Wuhu 蕪湖, and planned to prepare a large military campaign to re-conquer northern China. Instead of Yu Liang, Wang Dao was reinvested as regent.
A tax reform was carried out in 330, with the aim to levy taxes according to the size of fields. The ancient household tax system (hudiaofa 戶調法) of the Western Jin dynasty 西晉 (265-316) was given up. The government prohibited the unregulated seizure of wasteland by the local magnates and urged them to not surpass the quota of dependent households (see zhantian ). Violators of this law would be punished according to the law on robbery (qiangdao lü 強盜律). In the same year, new palace buildings were erected. In order to increase the number of appointees for state offices, the government issued an edict in 333 saying that each commandery governor (taishou 太守) should recommend potentials for state offices higher than ranked with a salary of 1,500 jin 斤 (see weights and measures).
In 336, Sima Yan was married with a girl from the house of Du 杜氏, a great-granddaughter of Du Yu 杜預 (222-285).
During that time, religious Daoism experienced a revival, and new schools emerged. At the same time, literati and intellectuals preferred discussing philosophical matters in so-called "pure conversations" (qingtan 清談), while many of them tended to interpret Confucianism by Daoist means. This philosophical mode is called "school of the mystery" (xuanxue 玄學). In 337, a National University (taixue 太學) was founded, where the Confucian Classics were to be interpreted in new ways.
When Sima Yan came of adulthood, he became very interested in political matters and was more and more unhappy about Yu Liang's style of government. He was outraged when he learnt that in 326 Yu Liang had killed Sima Zong 司馬宗, the Prince of Nandun 南頓. Confronted with the emperor's charge, Yu Liang argued that Sima Zong had plotted against the court. Sima Yan answered with a rhetorical question: "If you, Uncle, say that someone rebels, you kill him. Yet what to do if someone accuses you of planning a rebellion?"
In 342, Sima Yan fell ill and, having no issue, made Sima Yue 司馬岳 his successor.
He was succeeded by his oldest son Sima Yan, who is known as Emperor Cheng 晉成帝 (r. 325-342).