Amorphous MgNi+x%B(x=0,2,5,10) hydrogen storage alloys have been prepared by mechanical alloying (MA) and tested as hydrogen storage electrodes. The addition of boron can promote the MgNi alloy to form amorphous phase, increase the atom ratio of Ni/Mg at the alloy powders’ surface, and improve the thermal stability of MgNi alloy. The discharge capacity, the high rate dischargeability (HRD) and the exchange current density of the alloy electrodes all increased first, and then decreased with increasing the addition of boron. The maximum discharge capacity, the HRD400 and the exchange current density of the alloy with 5%B were 411.6mAh/g, 58.3% and 293.5mA/g respectively, which were 21.8%, 40.0% and 351.5% higher respectively, comparing with that of no boron addition. The electrochemical cycle stability of the alloy electrodes increased with increasing boron additions, and compared with no boron addition, the cycle capacity retention rate S20 of the alloy with 10%B increased 64.2%.