The potential transformer ferroresonance often occurs in neutral non-grounding distribution network，which cause zero sequence voltage rise. The phenomenon is similar to single-phase earth fault，and it may result in maloperation of line-selecting earthing devices. A new method is presented for discriminating between ferroresonance and single-phase earth fault. First，sampled data of zero-sequence voltage is fitted to sine function of 50 Hz，and it can get that the zero sequence voltage is power frequency or not by analyzing the amplitude. Second，to the case that zero-sequence voltage is power frequency，the fundamental frequency resonance can be correctly identified from single-phase earth fault by calculating waveform distortion level as zero-sequence voltage is distorted if fundamental frequency resonance occurred. Simulation data show that the method can discriminate single-phase earth and ferromagnetic resonance effectively.