A new method has been devised to obtain a Zn coating on the AZ91D magnesium alloy from the base electrolyte of ZnSO4 and K4P2O7 by electroplating. To improve the properties of the coating, a SnO2 sol was prepared and added in the electrolyte. The effects of the SnO2 sol addition on the morphologies, compositions and phase structures of the plated coatings were investigated by scanning electron microscopy (SEM), energy dispersion spectrometry (EDS) and X-ray diffraction (XRD). The results reveal that the addition of SnO2 in the solution changes the polarization behavior of the electrolyte and the compositions of the plated coating. The plated coating formed in the electrolyte with an addition of SnO2 sol are more uniform and provide a better protection for the AZ91D substrate.