Islanding detection is an essential function in the distributed generation system. According to defects of large detective blind area or power quality impact in the method of traditional distributed generation islanding detection, this paper proposed a passive islanding detection method based on extension pattern recognition. In this method, through the inverter side harmonic impedances measured for training, can successfully distinguish islanding effect, even under the matching of inverter output power and load power. Finally, through the experimental platform and MATLAB off-line simulation, verifies the correctness of the method.