Abstract: For problem that underground unbalanced condition will result in poor control performance of STATCOM, affecting compensation effect, the paper proposed a control strategy adopting improved switch function method and negative sequence voltage feedforward, made the STATCOM issue a negative sequence voltage whose magnitude and phase are equal to negative sequence voltage of the system to compensate for the negative sequence voltage of the system, so as to ensure normal operation of STATCOM under unbalanced condition. Simulation results show that the control strategy can inhibit negative sequence current, and effectively overcome impact of three-phase unbalance.