The embodiment of the invention provides a service life evaluating method and device for a sub-module capacitor of a modular multi-level converter, and relates to the technical field of power transmission. The problem that the service life of the capacitor in the MMC cannot be accurately analyzed in the prior art is solved. The method comprises the steps that the initial damage degree and servicelife of the capacitor in a sub-module of the MCC are initialized; the operating parameter of the MCC, a task profile parameter, sampling time interval delta t, the rated capacitance value Cd of the capacitor, the first aging correction factor kC of the capacitor, and the second aging correction factor kESR of an equivalent resistor ESR of the capacitor are acquired; the steps from S1 to S4 are circulated; if Dq is greater than or equal to 1, circulation is stopped; and the service life evaluation result of the capacitor is generated according to the service life Life of the capacitor, the sampling time interval delta t and the number of times q of circulation. The method and device are used for calculating the damage degree of the capacitor in the MMC.