When the grid fails, two methods are used for the islanding detection, one is the passive detection method and the other one is the active detection method which includes the active frequency drift, sliding mode frequency shift and output power disturbance method, but they all have non-detection zone. For the defects of these methods, this paper puts forward a new active islanding detection method. It identifies the islands through changing the size of the inverter output load with pulsating load, and detecting inverter output voltage and frequency. The theoretical analysis shows that this method is fast and has no effect on the grid, and there are not blind spots. This method also applies to multiple grid-connected PV inverters. Simulation results verify the validity of the method.