It is very important for a distributed generating system to have an effective islanding detection component.A new and effective islanding detection scheme is proposed.The scheme includes a thyristor placed between any two phases just by the port of a distributed generator（DG）.An artificial short time phase-to-phase short circuit is formed by triggering the thyristor periodically，so the short-circuit current and voltage pulse can be extracted.Because of this，the frequency response of system can be acquired according to the theory of harmonic impedance，so that the impedance measured from DG port can be obtained.Whether the DG is islanding can be determined quickly by monitoring the changes of this impedance.At the same time，disturbances between different DGs detecting signals are analyzed.This method is proved to be correct and effective by theoretical analysis，simulation and mockup experiment.