Prior to reign
Sima Yue was born in 322 as the second son of , by his wife Empress Yu Wenjun. After his father died in 325 and was succeeded by his brother , Sima Yue was created the Prince of Wu in 326. In 327, because his uncle the Prince of Langye wanted to yield that more honorific title, Sima Yu was created the Prince of Kuaiji and Sima Yue was created the Prince of Langye. It is not known where Sima Yue was during the Su Jun Disturbance of 326-328 -- whether he was captured and held hostage like his emperor brother, at his principality of Wu , or elsewhere. During his brother's reign, he received a progression of offices, but did not appear to actually participate in major decision-making.
In summer 342, Emperor Cheng grew gravely ill. He had two young sons -- and , then still in cradles, by his concubine . Yu Bing , one of the top two officials and Emperor Cheng's uncle, fearful that the Yus would lose power if a young emperor were named, persuaded Emperor Cheng that in the face of the powerful enemy Later Zhao that an older emperor should be named. Emperor Cheng agreed and designated Sima Yue the Prince of Langye, despite the other top official He Chong 's opposition. He died soon after, and Sima Yue took the throne as Emperor Kang.
In early 343, Emperor Kang created his wife empress.
Later in 343, Emperor Kang's other uncle, Yu Yi , proposed a major military campaign against Later Zhao, in coordination with Former Yan's ruler Murong Huang and Former Liang's ruler , both nominal Jin vassals. Most officials feared this large undertaking, but with support from Yu Bing, Huan Wen , and Sima Wuji the Prince of Qiao, Emperor Kang approved the plan and mobilized troops. Yu Bing was made a governor to coordinate with Yu Yi, and He Chong was recalled to replace him, and served in conjunction with Emperor Kang's father-in-law Chu Pou . However, Yu Yi eventually, for reasons unknown, largely did not carry out his campaign, although he made some border attacks.
In fall 344, Emperor Kang grew ill. Yu Bing and Yu Yi wanted to support his uncle Sima Yu the Prince of Kuaiji as emperor, but He Chong, consistent with his prior advice to Emperor Cheng, suggested that Emperor Kang should pass the throne to his son . Emperor Kang agreed and created Sima Dan crown prince. He died two days later, and was succeeded by the one-year-old Crown Prince Dan .
* ''Jianyuan'' 343-344
** Emperor Ming of Jin
** Empress Yu Wenjun
** Empress Chu Suanzi
** Sima Dan , the Crown Prince , later Emperor Mu of Jin