In order to solve the problem that the traditional methods are unable to ensure both power quality and the efficiency of detection, a new anti-islanding detection method is proposed based on the analysis of the relationship between the frequency and power-factor angle when islanding happens. The new method combines the varieties of frequency and the power-factor angle when islanding happens, which improves the sensibility of the criterion and enhances the reliability by delaying. Simulation proves that compared to the traditional method, the new method could detect islanding efficiently without affecting the power quality when the DG system is connected to the grid.