Islanding detection is one of the most important functions in the photovoltaic power generation system. In order to eliminate the non-detection zone and minimize the effect on the power quality， a novel islanding detection method using the positive feedback between reactive current and frequency is proposed based on the phase-locked loop of three-phase photovoltaic grid-connected inverter. The parameters are selected to guarantee the positive feedback and the flowchart of this method is given out. Simulation and experiments show that the method almost has no disturbance on the system under normal operations. But when an islanding occurs， the positive feedback between the reactive current and frequency will change the frequency quickly in one direction until triggering the islanding protection.