Abstract: Soluble magnesium alloys, as downhole tool materials, face problems such as poor corrosion resistance and low controllability of dissolution rate during service in harsh environments. Micro-arc oxidation process was used to prepare oxide film on the surface of AZ81 alloy. The effects of voltage, frequency and duty cycle process parameters on the corrosion resistance of micro-arc oxidation film were analyzed by characterization of micro-morphology, XRD, film thickness and electrochemical experiments. The results show that when the micro-arc oxidation voltage is 350 V, the frequency is 500 Hz and the duty cycle is 10%, the thickness of the micro-arc oxidation film is 15.2 μm and the surface is uniform; the corrosion potential is -1.39 V, the corrosion current density is 1.17 μA·cm^-2, the polarization resistance is 16054.64 Ω·cm², and the charge transfer resistance is 5012 Ω·cm². The corrosion current density is reduced by two orders of magnitude compared with the substrate, and the corrosion resistance of the micro-arc oxidation film is greatly improved.