Due to its advantages of high output level, stricter withstanding voltage and good wave quality, modular multilevel converter(MMC) has a widely research and applied in flexible direct current transmission engineering. The balance of sub-modular capacitor voltage has always been one of research focus in the field of MMC. Traditional capacitor sorting voltage algorithm will perform the up-and-down arrangement of the capacitor voltage in real time and select appropriate sub-modules according to the direction of the leg current in the next time. However, the increasing number of MMC sub-modules increases will cause higher switching frequency and larger switching losses. This paper puts forward an optimized sorting algorithm for capacitor voltage balancing. This strategy, considering the switching state of sub-module at the last moment, set the related threshold voltage to reduce switching operation times and the losses. Finally,this paper builds the MMC module in MATLAB/SIMULINK simulation platform to simulate and verify the algorithm. The simulation results shows that the effectiveness and good rela time performance of this prospered algorithm.