The invention provides an NLM-based optimization calculation method for an MMC loss aimed at an on-state loss and a switching loss of a nearest level modulation (NLM)-based modular multilevel converter (MMC), and an optimization method is embedded to improve the operation mode of the converter, thereby achieving the targets of energy conservation and loss reduction. The on-state loss is calculatedthrough calculating the switching number of submodules at each switching moment. For the switching loss, MATLAB programming calculation is carried out according to the basic operation principle of the modular multilevel converter. The validity and the accuracy of the calculation method are verified through a simulation system. The MMC loss under any working condition can be calculated and calculation is convenient and accurate; and in addition, appropriate operating parameters can be calculated for different operating conditions, so that the converter loss is minimal.