Abstract: The effect of impure ions on the performance of the solid polymer electrolyte water electrolyser（SPEWE）was studied. While impure ions were added into the anode compartment of the SPEWE, the variations of cell voltage, anode and cathode potentials with electrolysis time were analyzed respectively.The investigations revealed that these impure ions did great harm to the performance of the SPEWE.The rapid increasing cell voltage was mainly due to the increase of cathodic overvoltage.Ni2+,Cu2+electrodeposited at the interface between the cathode catalytic layer and Nafion membrane, and covered cathodic catalysts and decreased reaction active sites.Ca2+,Mg2+ formed a layer of hydroxide ,which had so bad electron conductivity that it not only covered cathodic catalysts but also increased interface contact resistance, resulting in rapid increase of cathodic overvoltage.