Plating processes greatly affect the corrosion resistance of electroless Ni–W–P coatings on magnesium alloys. In the present research, the corrosion resistance of electroless Ni–W–P alloy-coated AZ91D magnesium alloy was optimized by the response surface methodology. The optimum technological parameters of the plating process were determined by establishing a quadratic regression model, and the influence of these variables and their interactions on the corrosion resistance of the coating was analyzed. The optimum technological parameters of the electroless plating process were nickel sulfate concentration = 20 g/L, sodium tungstate concentration = 15 g/L, sodium hypophosphite concentration = 30 g/L, bath temperature = 60 °C, and bath pH = 9.3. Under these conditions, the coating had the best corrosion resistance. Among the aforementioned five variables, sodium tungstate concentration had the most significant influence on the corrosion resistance of the coating. Different degrees of interactions among the variables greatly affected the corrosion resistance of the coating.