The CuxBy alloy and amorphous Mg50Ni50 hydrogen storage alloy were synthesized by mechanical alloying method (MA). And then surface modification of the amorphous Mg50Ni50 alloy was done using CuxBy by the same method. Effects of CuxBy with different composition and different doping ratio of CuxBy (the same composition) on the cycle stability of Mg50Ni50 alloy electrode were investigated. Results show that the surface modification through CuxBy with different composition can increase the cycle stability of Mg50Ni50 alloy electrode to different extent. However, the surface modification effects of CuxBy with the same composition but different doping ratio on the cycle stability of Mg50Ni50 alloy electrode have obvious difference. When Mg50Ni50:Cu0.9B0.1=5:1, the initial discharge capacity was 465 mAh·g-1, and the 50th one was 222 mAh·g-1. On the condition of keeping high discharge capacity, the cycle stability of amorphous Mg50Ni50 alloy was improved effectively.