The active frequency drift（AFD） method can be used to detect the islanding status for distributed grid-connected power system. However， when the angle of the current disturbance matches with the load angle in the non-detecting zone（NDZ）， the AFD method cannot identify the islanding. In order to solve the above problem， a novel islanding detection method based on the alternate current disturbances is proposed. In the case that the disturbed current is in phase with the voltage at the point of common coupling（PCC）， the current frequency shifts up in the previous cycle and shifts down in the current cycle. The islanding is detected by judging whether the voltage frequency at the PCC shifts up and down alternately. Simulation and experimental results verify that the proposed method can not only identify the islanding very well， but also solve the NDZ problem of the AFD method.