Power quality waveform distortion detection is one of the most important works for power quality diagnosis. It is the basis to decide when to record the fault power quality signals. This paper presents a new method to detect waveform distortion by similarity measure. 6 types of typical power quality events are simulated to compare the effect of distortion detection. Simulation results show that the distance measure is suit to waveform distortion detection and normalized distance has the best effect of detection.