The effects of potential on the surface morphology, chemical composition, corrosion resistance, and interfacial contact resistance of 316L stainless steel (316LSS) in a mixed solution of 0.5 mol/L KNO3 and 0.1 mol/L HNO3 were studied to solve the problems of corrosion and high surface contact resistance of 316LSS bipolar plate in PEMFC. The electrochemical reaction and properties of 316LSS were studied by electrochemical impedance (EIS), cyclic voltammetry (CV), chronoamperometry, and potentiodynamic polarization tests. Electrochemical workstation, scanning electron microscope and X-ray photoelectron spectroscopy were used to characterize the corrosion resistance, microscopic morphology, and elemental valence of 316LSS, and the interface contact resistance and the concentration of Fe, Cr metal ions in the solution were measured. The reaction on the 316LSS surface is irreversible in the mixed 0.5 mol/L KNO3 + 0.1 mol/L HNO3 solution. EIS showed that there is an inductive impedance arc in the low frequency, and the 316LSS surface appeared to a convex structure under –0.5 V (vs. SCE). When the potential shifted to –0.6 V (vs. SCE) and –0.7 V (vs. SCE), pitting corrosion, and grain boundary corrosion occurred, and the integrity of the 316LSS surface is destroyed. The element analysis shows that the key alloy element chromium mainly exists in the chromium oxide and chromium nitride, the chromium nitride accounted for 54.8% (–0.5 V (vs. SCE)). The minimum resistance is 8.7 mΩ.cm2 at 140 N/cm2 (–0.5 V (vs. SCE)). The corrosion resistance of modified 316LSS is significantly improved, and the corrosion current density and corrosion potential of the best sample are 0.065 μA/cm2 and 136.738 mV, respectively. The corrosion current density was 3.4 μA/cm2 after 650 h operation in the durability test. The physical and chemical properties of modified 316LSS are closely related to the applied potential. Due to the selective dissolution of the 316LSS passivation film during the electrochemical reaction, and the thickness is reduced. The nitrogen doping stabilizes the passive film and improves the conductivity, thus eliminating the adverse effect of the passive film on the performance of the 316LSS bipolar plate. Under the optimal modified potential, the surface of 316LSS was selectively etched to form a dense nitrogen-doped film, which improved the comprehensive properties of 316L stainless steel bipolar plate.