A static synchronous compensator (STATCOM) is used into distribution grid for compensating reactive power and three- phase unbalance load for the distribution grid. According to analyzing the positive sequence equivalent circuit and negative sequence equivalent circuit under the three-phase unbalance load of distribution grid, using the symmetrical component method and superposition principle, a new cascade loop control strategy of positive sequence current and negative sequence current is proposed. Using δ-θ control, the positive sequence loop compensates reactive power of distribution grid and maintains stability of STATCOM DC voltage. Using φ-θ control in the negative sequence loop can make STATCOM compensate negative-sequence current caused by three-phase load unbalance and keep distribution grid side three-phase load balanced. Simulation and experiment results are presented to verify the validity of the proposed control strategy.