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In order to improve the corrosion resistance of magnesium alloy, Ni-P coating was prepared on the surface of AZ91D magnesium alloy by chemical deposition method, and the optimum process of electroless Ni-P plating was determined. In this experiment, Scanning electron microscope (SEM) and Autolab electrochemical workstation were used to study hardness, microscopic morphology and corrosion resistance of the coating. The results showed that Ni-P coatings have better corrosion resistance in 3,5% NaCl solution under the condition: CeCl₃ 0,15 / g/L, NiSO₄ 28 / g/L, Na₂HPO₃ 25 / g/L, temperature 85 / °C and pH = 7,3. The corrosion rate of the coating decreases first and then increases with increasing amount of Cerium chloride.