Abstract: Investigates influences of pulses on the bottom gap,the side gap and the electrolyte flow field in ultra-short group pulses(UGP) electrochemical machining.Models of the bottom gap and the side gap under group pulses and model of pressure wave under ultra-short pulses were presented.Based on the models the influence on the gaps and the pressure wave under UGP were discussed.The gaps become less under group pulses and the pressure wave become stronger with less gaps.The pressure wave becomes stronger under ultra-short pulses than that under high frequency pulses.In contrast with high frequency pulses electrochemical machining(ECM),the gaps between electrodes were less and the characteristics of the electrolyte flow field were improved in UGPECM,so the machining quality becomes improved.