Corrosion process of the anodized film on AZ91D magnesium alloy's surface in 3.5% NaCl aqueous solution was investigated with electrochemical impedance spectroscopy(EIS).According to the characteristic of EIS in the corrosion process,the EIS of anodized film at the initiation stage and at the propagation stage of pitting corrosion were simulated using the equivalent circuit of R(RQ)(RQ)and R(Q(R(RQ)))models,respectively.The results show that the solution resistance Rsol and the Yp of porous layer increased,and the resistance Rp of porous layer and the resistance Rb of dense layer decreased gradually.The np value almost unchanged and the Yb and nb of dense layer have no obvious regularity with prolonged of immersion time at the initiation stage of pitting corrosion.The solution resistance Rsol,the n1 value and the reaction resistance R2 in corrosion pit decreased,while the Y1 increased,the solution resistance R1,the Y2 and n2 of metal/medium interface for the anode in corrosion pit have no substantial regularity with prolonging the immersion time at the propagation stage of pitting corrosion.