The work aims to improve corrosion resistance of AZ91D magnesium alloy. Ni-Mn alloy film was electrodeposited on the surface of magnesium alloy in the choline chloride-urea ionic liquid by electrodeposition method. After homogenization pretreatment, the copper layer was electrodeposited on the surface of magnesium alloy to improve the adhesion between subsequent Ni-Mn alloy film and the substrate. The effects of Mn element on the surface morphology, chemical composition and phase structure of the film were studied by using scanning electronic microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction spectroscopy (XRD). Once Mn was incorporated into the film, the surface morphology transformed from pyramid-like to tumor-like clusters. Moreover, Mn content of Ni-Mn alloy film rest on current density and increased as the current density increased. Corrosion resistance of the coating could be improved by introducing a little Mn compared with pure Ni film. However, the corrosion resistance of the film was weaker than that of the pure Ni film when too much Mn was introduced. The Ni-Mn alloy film, when containing 3.078at.% Mn, shows the best corrosion resistance with self-corrosion current Jcorr of 0.301 μA/cm2 and corrosion potential Ecorr of ?0.157 V (vs. SCE).