Abstract: In order to establish the theoretical basis of self balancing of modular multilevel converter (MMC), the MMC state space model is constructed, and the uniform asymptotic stability of MMC capacitor voltage is proved. The static operating point of MMC capacitor voltage under unbalanced state is studied, and the correlation between the rank of switching state matrix and the static operating point of MMC capacitor voltage is revealed. Based on the self balancing theory of switching state matrix, to solve the problem of slow rotation speed when the control frequency is low or the number of sub modules is large, a capacitor voltage balancing control strategy based on grouping rotation of switching state matrix is proposed, which can effectively reduce the fluctuation rate of capacitor voltage. Finally, the MMC electromagnetic transient simulation model is built in Matlab/Simulink to verify the correctness of the verified conclusions and the effectiveness of the proposed method.