Islanding detection is an essential function of distributed grid-connected power generation systems, as the islanding occurrence is unpredictable and very harmful. The voltage and frequency of the constant-power controlled grid-connected inverter working in the islanding state has been studied. The none detection zone (NDZ) is calculated when the output of the inverter has reactive power. Then, intermittent bilateral reactive power variation anti-islanding strategy was proposed. The conditions to eliminate NDZ are presented and the variation parameters are designed. Finally, experiments results indicate that the proposed method could meet requirement and has little influence on power factor.