The electrical stress of high voltage and high current is the characteristics of the sub module base on MMC.And IGBT is the key component of MMC module.A calculating method for loss and junction temperature of IGBT base on MMC is very important to the heat design and type selection of IGBT in MMC engineering application.Firstly,the stress of the module under steady state operation of MMC is analyzed.Secondly,a method for calculating the IGBT loss and junction temperature of MMC module is given with current and sub module switching and junction temperature estimation model.Finally,the calculation method is verified in the back to back test system.The proposed method is proved to be effective and feasible.