The nano-sized Ni-SiO2 coating was prepared on AZ91 magnesium alloys by composite electroplating, and the effect of process parameter on the coating was studied and the optimum process was obtained. The results show that the surface of the coating is smooth and the coating posesses the best micro-hardness and corrosion resistance when the content of nano-sized SiO2 and activator is 20 g/L and 1 g/L in the electroplating bath with pH value 5~6, respectively, and the electrodepositing time is 20~30 min at 50℃ and the the cathodic current density is 1 A/dm2.