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Island detection in the distributed power generation system is the difficulty of the current research. In particular, the traditional active frequency drift method is difficult to detect the islanding phenomenon and the current-output?has large harmonics contents when the inverter output power and load active power balance or a small degree of imbalance. For this problem, an improved active frequency drift island detection method is proposed in PV grid-connected power generation system. This method combines the active frequency drift and Kalman filtering of harmonic estimation method to determine whether or not it is produced the islanding phenomenon. At the same time, by reducing the frequency detection threshold, the traditional active frequency drift method reduces effectively non-detection zone, and reduces significantly the inverter output current harmonic contents. Simulation of Matlab/Simulink is carried out to verify the effectiveness of the proposed method.