The invention relates to a method for calculating the loss of an MMC (Modular Multilevel Converter). The method comprises the following steps: (1) extracting a parameter; (2) interpolating the parameter to generate a figure and a table of conduction losses and switching losses; (3) simulating to obtain current and input environment temperature, and looking up the loss of a current time in the table; (4) calculating a junction temperature, and calculating a next node; (5) converging to obtain loss. By adopting the method, the calculation and simulation time can be shortened greatly, and a more real and reliable data source is provided in comparison to simple curve fitting. By adopting the method, real-time calculation of the internal junction temperature of an IGBT (Insulated Gate Bipolar Translator) module becomes possible; the method has high practical values in state monitoring of the IGBT module, estimation of the service life of the IGBT module, prolonging of the service life of an IGBT module device, and improvement on the reliability of the IGBT module device, and the like.