Summary This paper makes some systematic studies of the origination, the physical-mathematical characteristics and the disturbing function of the pressure wave in the inter-electrode gap, especially of the influence on the copying accuracy in PECM. Through the observation of high speed photograph to the real process in the gap and the analysis of precise measuring data for the pressure wave, we have recognized perceptually about the three respects. On the basis of the observation and analysis, a mathematical model of the gap pressure wave is deduced and verified, and by use of this model a simulated test by the digital computer is carried out. It is concluded that the gap pressure wave has a great influence on the copying accuracy in PECM.