It is very important for a distributed generating system to have an islanding detection part. Focused on the problem of islanding, this paper proposes a new and effective islanding detection scheme: a thyristor is placed between any two phases just by the port of the distributed generator (DG). An artificial short time phase to-phase short circuit is formed by triggering the thyristor periodically when the voltage between this thyristor is almost equal to zero, so the short-circuit current and voltage pulse can be extracted out. Because that short-circuit current and voltage pulse contain plentiful frequency components, 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 impedance. This method is proved to be correct and effective by theoretical analysis and simulation.