The electroless nickel plating on AZ31 magnesium alloy was studied, basing on the pretreatment process of H3PO4+HNO3 pickling and NH4HF2+H3PO4 activation. The morphology, composition distribution, phase structure and elemental analysis of electroless nickel plating coating were investigated through scanning electron microscope (SEM), energy dispersive analysis of X-ray(EDS), X-ray diffractometer (XRD), x ray photoelectron spectroscope(XPS). The results indicate that the surface of the electroless nickel plating coating is smooth and bright, the structure of deposits is amorphous with high content of phosphorous. The occurrence mode of nickel on the surface of deposits is pure nickel(26.7%), NiO(32.96%)and Ni(OH)2(40.29%), and occurrence mode of phosphorus is pure P(20.17%), PO42- (17.37%)and PO43-(62.46%). Furthermore, the occurrence mode of nickel in the intermediate layers of the coating is almost 100% pure nickel, and occurrence mode of phosphorus is pure P (74.11%) and PO42-(25.89%).