The work aims to improve corrosion resistance and expand application scope of LZ91 Mg-Li alloy. Based on pre-experiments, the effects of different formulations and processes of nickel plating on the process of electroless nickel plating, morphology, porosity, corrosion current density of Ni-P alloy coating were studied by means of orthogonal experiment. Optimized process conditions of nickel plating were confirmed: primary nickel acetate of 20 g/L, reducing hypophosphite of 20 g/L, complexing agent citric of 6 g/L, ammonium hydrogen fluoride of 20 g/L, hydrofluoric acid of 12 mL/L, a small amount of ammonia, temperature of 85 ℃ and pH=7.4. Admirable nickel coating was obtained in the optimized process conditions with the thickness of 5~7 μm, low surface porosity, self-corrosion potential of –0.3806 V (increased by 1.356 V compared with untreated Mg-Li alloy) and self corrosion current density of 1.97×10–7 A/cm2 (decreased by two orders of magnitudes). Owing to protection by the coating, the sample passed 96 h-salt spray test successfully and met military standards. Corrosion resistance of Mg-Li alloy can be improved by the coating made by taking advantage of optimized process.