In order to improve the wear resisting property of magnesium alloy’s surface coating, Al2O3 nano ceramic coating on the surface of AZ31 magnesium alloy is prepared using the magnetron sputtering process. Influences of temperature, power and time on hardness of surface layer are discussed by orthogonal method. Morphology, phases and composition of coatings are analyzed by SEM, XRD and EDS. The results show that the time and power are important factors to affect hardness and wear resistance. The process parameters of 80 ℃，200 W and 3 h are the optimal sputtering process. Through this process, the thickness of nano Al2O3 coating on AZ31 is about 2 to 10 μm and the grain size is about 30 nm. The surface hardness of ceramic coating reaches up to 1 360 MPa, and increases more than 77.31% of the substrate hardness of nano Al2O3 coating.