The work aims to improve the anti-corrosion properties of the electroless Ni-P coatings on magnesium alloys. Ni-Cu-P/Ni-P composite coatings were prepared by plating Ni-Cu-P and Ni-P on AZ31B magnesium alloys. The change rules of surface morphologies, compositions, thicknesses and corrosion current densities of the composite coatings along with the CuSO4 concentrations in the bath solutions were studied. The cross-section morphology, compositions and crystal structure of the com-posite coating with CuSO4 concentration of 1.0 g/L were characterized. Potentiodynamic polarization curves and salt spray tests were conducted to investigate the anti-corrosion properties and mechanisms of the composite coatings. From the experimental results, the copper content in the composite coatings increased with the increasing CuSO4 concentrations, and copper had sig-nificant influences on the structures and properties of the composite coatings. The copper codeposition could enhance the com-pactness of the composite coatings to a large extent by suppressing the growth of the nodules and producing more nuclei. As the concentration of CuSO4 increased, the catalytic activity on the sample surface decreased and the stability of bath solution improved, which resulted in the obvious decrease in the coating thicknesses with the increasing CuSO4 concentrations. The composite coating with a CuSO4 concentration of 1.0 g/L was uniform, compact, and passivable, and the corrosion performance was found to withstand >180 h salt spray test in accordance with ISO 9227. Ni-Cu-P/Ni-P composite coatings can provide a superior corrosion resistance to the magnesium alloy substrates. The passivable and compact coating layers of composite coatings are main factors to improve the anti-corrosion properties.