To improve wear resistance of the surface of magnesium alloy, laser alloying of Al-Cu powders coating on AZ31B magnesium alloy is achieved by the method of replaced-powder. The composition microstructure of the coating are analyzed by X-ray diffraction (XRD) and scanning electric microscope (SEM). Surface properties of the coating are evaluated using a micro-hardness tester and a abrasion tester condition at room temperature. The results show that the Al-Cu alloying layers are well bonded with the substrate; The XRD patterns show that the alloying coating is composed of α-Mg, β-Al12Mg17 and CuMg2, and the cortent of β-Al12Mg17 is higher than that of substract. The average micro-hardness of the surface layer is significantly improved from 50 HV to 210~265 HV, 4~5 times higher compared to the AZ31B substrate and the relative wear resistance is 2.5.