Abstract: During long-term operation,the components（such as bipolar plates and elastomeric gaskets）of proton exchange membrane fuel cell（PEMFC）may degrade and produce the metal ion contaminants(e.g. Ca²⁺,Mg²⁺). This could affect the long-term stability and durability of the PEMFC for its applications. In this paper,polarization curves and electrochemical impedance spectroscopy（EIS）were investigated experimentally by introducing the various concentration（i.e. 100 mg/L,300 mg/L and 500 mg/L）Mg²⁺solutions into the cathode air stream of the PEMFC,and then the effects of Mg²⁺contaminant at the cathode on the PEMFC electrochemical performance and electrochemical impedance were analyzed. The polarization curve results show that the cathodic Mg²⁺contaminant with three levels of concentrations had significantly negative effects on the electrochemical performance of the fuel cell,the current density of the fuel cell contaminatedbyMg²⁺decreased,andthecurrentdensitydecreasedmoredrasticallywiththeincreaseofMg²⁺concentrationand contamination time. The theoretical analysis results of polarization curves indicate that the cathodic Mg²⁺contaminant with three levels of concentrations could affect the permeability of the proton exchange membrane and increase the H+impedance in the proton exchange membrane,and the performance of the proton exchange membrane deteriorated more severely with the increase of contamination time. The EIS results show that the cathodic Mg²⁺contaminant with three levels of concentrations could increase the H+impedance in the proton exchange membrane and the cathode Faraday resistance,and the H+impedance in the proton exchange membrane and cathode Faraday resistance increase with the increase of contamination time,and the cathode electric double-layers are not significantly affected by the Mg²⁺contaminant at the cathode in this work.