The polypyrrole (PPy) coating and polypyrrole-molybdate (PPy-MoO42-) coating were electro-polymerized in sodium salicylate solution by using cyclic voltammetry (CV) on the AZ31 Mg alloy surface. Surface morphology of PPy coating and PPy-MoO42- coating on Mg alloy surface before and after being corrupted was observed by scanning electron microscopy (SEM). Attenuated total reflection-infrared (ATR-IR) spectra showed the characteristic ring stretching peaks of the PPy coating and PPy-MoO42- coating, and also revealed the influence of MoO42- on the characteristic ring stretching peaks for PPy. The surface resistance was measured by four-point probe method. The open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization analyzed the corrosion resistance performance of PPy coating and PPy-MoO42- coating on the Mg alloy surface in 3.5 % NaCl solution for 12 h. The results showed that the surface morphology of PPy-MoO42- coating is compact. And the corrosion current density of PPy-MoO42- coating is three orders lower than the PPy coating, the PPy-MoO42- coating on the Mg alloy surface had better corrosion resistance and smaller surface resistance than the PPy coating because of the existence of the molybdate ion.