Abstract: The surface microstructure and phytic acid conversion film surface morphology of AZ31 cast and rolled magnesium alloy sheet were observed by SEM and Leica optical microscope. The potential-time curve of phytic acid conversion film during forming process on casting and rolling plate was monitored by electrochemical workstation. The effects of conversion time on the surface morphology and corrosion rate of the conversion film were analyzed by OM and polarization curve. The forming mechanism of the cast and rolled magnesium alloy conversion film was studied and analyzed. The results show that, the formation process of the conversion film mainly includes four stages: the rapid dissolution, the early stage of film formation, the middle stage of film formation and the later stage of film formation. The conversion film is mainly composed of a large number of isometric reticulated films. The size of reticulated film decreases with the increase of treatment time, while the width between reticulated films increases. The corrosion rate of the conversion film decreases at first and then increases with the prolongation of treatment time. This is related to the structure and compactness of the conversion membrane. The anodic dissolution process is mainly pore corrosion and filamentous corrosion, which is mainly related to the microcrack defects of cast and rolled plate.