The sub-module's（SM's）capacitor voltage balancing problem of modular multilevel converters（MMCs）is one of the technical difficulties demanding prompt solution in the MMC engineering application.In contrast to the traditional voltage balancing method which uses the sorting results of SM capacitor voltages according to the direction of the arm current，this paper proposes an improved and optimized method by introducing the double holding factor to reduce the switching frequency and losses.Meanwhile，the voltage balancing strategy of packet sorting is adopted in the proposed method to maintain the relative balance of energy between the SMs by introducing the energy balancing factors on the premise of ensuring normal operation of MMC.The optimized voltage balancing method has realized reduction of the sorting operand and improvement of the capacitor voltage sorting speed.The model of MMC with ten SMs is realized by PSCAD/EMTDC，with both correctness and effectiveness of the proposed method verified by simulation results.