Abstract: In order to develop green manufacturing and microfabrication, an innovative electrochemical machining method was introduced, that is the electrochemical machining(ECM)technology in ultra-pure water.The relative relationship of water dissociation and current density under the function of strong acid cation exchange membrane was studied, and the mechanism of electrochemical machining in ultra-pure water was also explored.On the basis of theoretical analysis, the significance of two turning points of the voltage-current density characteristic curve was presented.As the result of water dissociation and machining experiments, the factors affecting current density were determined.