Emperor Jin Huidi 晉惠帝 (r. 290-306), personal name Sima Zhong 司馬衷, courtesy name Zhengdu 正度, was the second emperor of the Jin dynasty 晉 (265-420). He was the second son of Sima Yan (Emperor Wu 晉武帝, r. 265-290) and was born by Empress Yang 楊皇后. After the early death of his older brother Sima Gui 司馬軌, Sima Zhong was appointed Heir Apparent (huang taizi 皇太子) in 267, yet because of his lacking intelligence, Emperor Wu desired to choose another son as his successor. Empress Yang successfully defended Sima Zhong's legal claim to the throne.
According to the last will of his father, Sima Zhong made his grand-uncle Sima Liang 司馬亮 (d. 291), Prince of Runan 汝南王, as well as his grandfather Yang Jun 楊駿 (d. 291) regents. His own empress was Jia Nanfeng 賈南風 (257-300, Empress Jia 賈后), and their common son Sima Yu 司馬遹 (278-300) was given the title of Heir Apparent (huang taizi 皇太子). Sima Zhong chose the reign motto Yongxi 永熙 "Eternal Brilliance" and gave those among the high state officials (salary 2,000 shi 石 and more, see also weights and measures) which did not yet bear a title of nobility, that of Marquis within the Passes (Guanzhong hou 關中侯).
Yang Jun was ambitious enough to drive Sima Liang out of office, and thus was the sole powerful regent of the Jin empire. This situation was the initial spark for the rebellion of the Eight Princes (ba wang zhi luan 八王之亂) which contributed substantially to the downfall of the Western Jin 西晉 (265-316). In 291, Empress Jia conspired with Sima Wei 司馬瑋 (271-291), Prince of Chu 楚王, to assassinate Yang Jun, Sima Liang and Wei Guan 衛瓘 (220-291). The empress took over regency herself. She not only appropriated much wealth for her own kinsmen, but also deprived Sima Jun of the title of Heir Apparent. A year later, the Prince of Zhao 趙王, Sima Lun 司馬倫 (240-301), and the Prince of Qi 齊王, Sima Jiong 司馬冏 (d. 302), conquered the capital Luoyang 洛陽 (today in Henan) and killed Empress Jia. Sima Lun then took over regency over the empire with the titles of Counsellor-in-chief (xiangguo 相國), commissioner with special warrant (shichijie 使持節) and supervisor of all armies (dudu zhongwai zhujun shi 都督中外諸軍事).
In 301, Sima Lun dethroned Sima Zhong and adopted the title of emperor. A group of princes thereupon decided to overthrow the usurper and re-enthroned Sima Zhong. His supporters were Sima Jiong, Sima Ying 司馬穎 (279-306), Prince of Chengdu 成都, and Sima Yong 司馬顒 (d. 306), Prince of Hejian 河間. The emperor was from then on just a puppet of various prince regents. He was even abducted to Ye 鄴 (today's Linzhang 臨漳, Hebei) and then to Chang'an 長安 (today's Xi'an 西安, Shaanxi). In 304, the title of imperial brother (huang taidi 皇太弟, which was close to that of Heir Apparent) was bestowed on Sima Chi 司馬熾 (284-313), the Prince of Yuzhang 豫章.
Not only was the empire afflicted by war between the princes. Draught and famine added to the precarious political and economic situation of the Jin empire. Sima Zhong was perhaps an imbecile (chidai 癡呆) and died in 306 without an issue. He was buried in Mound Taiyang 太陽陵 close to Luoyang and was given the posthumous title Emperor Xiaohui 孝惠皇帝 "Filial-Belevolent". Some sources say he was poisoned by Sima Yue 司馬越 (d. 311), regent and Prince of Donghai 東海.
Emperor Hui was succeeded by his younger brother Sima Chi, who is known as Emperor Huai 晉懷帝 (r. 306-311).