To improve corrosion resistance of the bipolar plates of proton exchange membrane fuel cell (PEMFC) at operating conditions, PANI/316L SS bipolar plates were prepared by electro-polymerization of polyaniline (PANI) thin films on 316L stainless steel (SS) by cyclic voltammetry method using 1-ethyl-3-methyl imidazole ethyl sulfate (EMIES) ionic liquid as the polymeric electrolyte. SEM was used to observe the surface morphology, FTIR was used to analyze the structure of functional groups, and XPS was used to analyze the element composition and bonding state. The potential of PANI/316L SS was controlled at 0.5, 0.6 and 0.7 V (vs. SCE) respectively by constant potential method to simulate PEMFC cathode potential, and the changes of open circuit potential, polarization curve and electrochemical impedance spectrum (EIS) after constant potential polarization were measured. The results show that there are micro-cracks in PANI films. FTIR shows benzene ring, quinone ring, S=O and —COOH stretching vibrations. XPS indicates that PANI film contains C, N, O and S elements. The current density of PANI/316L SS polarization curve fluctuates greatly in the passivation area. Nyquist plots of EIS are composed of high frequency reactance arcs and low frequency straight lines. The radius of reactance arcs increases with the increase of polarization potential. It is found that PANI shows intermediate oxidation state structure, both EMIES anion (CH3CH2SO4–) and oxalic acid anion (HOOC-COO–) is doped in the PANI molecular chains. PANI/316L SS is in anodic polarization state under PEMFC cathode working potential, and the potential has significant influence on corrosion resistance of PANI/316L SS, PANI/316L SS shows good corrosion resistance at 0.6 V, and excessive oxidation of PANI occurs when the potential increases to 0.7 V, which results in de-doping of the “counter anion” and decreasing of conductivity and corrosion resistance of PANI/316L SS.