Objective To improve the surface corrosion resistance of AZ91D magnesium alloy by Mo-Mn chromium-free conver- sion coating. Methods Effects of different concentrations of NaMoO4 and KMnO4 and temperature on the transformed sample film were studied by orthogonal experimental method. After optimizing experiment parameters, the effects of the time on the conversion film was also studied. The morphology and component of conversion coatings were researched by SEM and EDS. Moreover, the po- larization curves and electrochemical impedance spectroscopy of conversion coating were tested in 3. 5% NaCl aqueous solution. Results The results showed that when the condition was 10 g / L NaMoO4 , 6 g / L KMnO4 at 50 ℃ and pH 5 for 40 min, where the morphology of the sample was relatively even and the amount of cracks was relatively low. Compared to the corrosion resistance of magnesium substrate, the corrosion potential of conversion film was increased by about 0. 075 V ,and the corrosion current density decreased by nearly 1 order of magnitude. When the condition was 20 g / L NaMoO4 , 8 g / L KMnO4 at 50 ℃ and pH 5 for 40 min, the morphology of the sample was the most even and the amount of cracks was the least. Compared to the corrosion resistance of magnesium substrate, the corrosion potential of conversion film was increased by about 0. 047 V ,and the corrosion current density decreased by nearly 2 orders of magnitude. EIS showed that the polarization resistance of the latter conversion film sample was 1450. 2 Ω, while the polarization resistance of magnesium matrix was 806. 4 Ω. Conclusion Mo-Mn chromium-free conversion coating could obviously improve the surface corrosion resistance of AZ91D magnesium alloy.