The active probability of upper and lower controllable switches of a modular multilevel converter（MMC）is analyzed under sinusoidal pulse width modulation.By referring to the active probability，the equivalent current of the upper or lower controllable switch can be derived.Then，based on voltage variation of the submodule capacitor in each fundamental period being zero，the voltage of the submodule capacitor can be obtained according to the capacitance of the capacitor，power factor，modulation ratio，AC current peak on valve side，and so on.And based on the equivalent current of the upper or lower controllable switch，the average current and effective current can be derived，respectively.Using the related parameters of the power losses of the controllable switch provided by the manufacturer，the power losses and junction temperatures of the controllable switch of Xiamen voltage source converter based high voltage direct current（VSC-HVDC）demonstration project are calculated under rectification operation and inverter operation conditions of the MMC，respectively.The results of the loss evaluation model and junction temperature evaluation model show that the evaluation models are able to provide the MMC cooling system configuration with reference.