In three-phase islanded microgrid systems, unbalanced loads will cause negative-sequence current and negative-sequence voltage. The difference between line parameters will seriously affect the distribution of negative sequence current between micro sources and reduce the reliability of the microgrid systems. Besides, the bigger line impedance in low voltage microgrid systems will increase the unbalance of the microgrid systems. To solve these problems, this paper proposes a new islanded microgrid control strategy based on PR controllers under unbalanced loads. By controlling positive-sequence and negative-sequence independently, the strategy can realize the design of negative-sequence virtual impedance to reduce transfer impedance of the micro sources, suppress the negative-sequence circulating current and reduce unbalanced degree of the microgird systems. Simulation by Matlab/Simulink results demonstrate that the control strategy is correct and effective. This work is supported by Natural Science Foundation of Shanghai (No. 16ZR1413000).