Anodized films were prepared on AZ31 magnesium alloy. The forming process of the anodized films was studied by electrochemical impedance spectroscopy (EIS) measurements. The results show that a significant regularity appears among electrochemical impedance spectra for oxide films obtained from various oxide time; their forming can be divided into four stages. There are significant differences among stages. The equivalent electrical circuit R(C(R (QR))) (CR) was used to analyze the EIS data. The fitting results show that the number of pores and roughness of the film increase with increasing of the oxide time. The growth of barrier layer and porous layer is not synchronized while their growth is in the dynamic change.