A three-phase four-leg topology is used in microgrid converter because of its advantages in unbalanced networks. To improve the islanded-mode power quality of microgrid system under unbalance load conditions, a modified control strategy using decomposed sequence components is presented. Positive and negative sequence components are extracted by a second order generalized integrator based detection method and controlled in positive and negative synchronous rotating reference frame respectively. A proportional resonant regulator is adopted in independent zero axis controller. It enhances the ability to suppress the zero sequence voltage and simplifies the control algorithm. Finally, the control strategy is verified by simulation and experiments. The results show that the proposed method is capable of maintaining the symmetrical output voltage under unbalanced load conditions and the performance of islanded-mode is enhanced.