Due to its unique topology, the STATCOM based on MMC has many advantages, but this unique topology also makes the voltage balancing control of DC side capacitors more difficult. Aiming at the DC side capacitor voltage unbalancing of STATCOM, through the analysis of MMC STATCOM mathematical model, decoupling separation of the active and reactive based on the decoupling transformation, a strategy of DC voltage control and voltage-sharing based on sort algorithm to balance the DC side capacitor voltage is proposed. The steady value of capacitors voltage and active power exchange are determined by the DC voltage control and the capacitor voltage balanced by voltage-sharing based on sort algorithm. Simulation results show that this method can effectively stabilize the DC side’s voltage of the MMC STATCOM.