In order to achieve reactive and negative sequence compensation under unbalanced load conditions,static synchronous compensator based on modular multilevel converter (MMC-STATCOM) is used to compensate the reactive power and the three phase unbalance simultaneously.Taking the positive and negative sequence separation control as the core idea,the traditional double coordinate system control method is improved,and the positive and negative sequence components are controlled in DQ and the alpha beta coordinate system respectively.Compared with the traditional method,reduced structural complexity and increased control flexibility.A bipolar control scheme is proposed based on the direct current control method.The simulation results verify the correctness and effectiveness of the improved method and comprehensive compensation control strategy proposed.