According to the History of Yuan, he died of alcoholism. However, it may not have been as simple as merely drinking too much. It also states that shortly before his death, some ministers of the court wanted to propose that Kublai Khan abdicate his throne to Zhenjin on account of old age and because Prince Zhenjin was highly respected throughout the empire. However, Prince Zhenjin tried to prevent this from happening. Unfortunately, Kublai Khan found out anyways and was furious, which terrified Zhenjin and may have led him to overdrink

Song Lian, Wang Yi, et al. 宋濂 王禕 等撰. "Yuan Shi" 元史 [History of Yuan]. Taiwan shangwu yinshuguan 臺灣商務印書館 "The Commercial Press, Ltd.", 2010.