The kinetics of film-forming process of anodizing on AZ91D Mg alloy have been studied through analyses of voltage-time and thickness–time curves. The surface morphology, structure, composition and valence of element, phase constituent of anodic films have been analyzed by SEM, EDS, XPS and XRD respectively. The results show that the film-forming process can be divided into four stages: I - formation of a dense layer; II - formation of a porous layer; III - fast growth of the porous layer; IV - slow growth of the porous layer. The growing process of the dense film is characterized by the rapidly formed intact and imperforate oxide film; there are few micro-cracks in partial film formed on α phase, on the contrary, intact film is formed on β phase; the forming of porous film is based on the particulate sparking product with micro-porous structure. There are two kinds of pores in the anodic film; the first one is circular or ellipse, while the second one is the irregular gaps. The dense layer is composed mainly of MgO and small amount of Al2O3, while the porous layer mainly consists of MgSiO3 and Mg3B2O6. We propose a possible model about growing anodizing film on AZ91D Mg alloy based on the experimental results.