Abstract: This paper studied the effect of B additive on the electrochemical properties of hydrogen storage alloys with the Co mass fraction less than 5% and designed non-stoichiometry cast alloys Mm(NiCoMnAlB)_5+χ(χ=0.02, 0.03, 0.05, 0.1, 0.2, 0.3). The results show that the activation performance and high ratio discharge property are improved, but the discharge capacity is reduced. Microstructure analysis approves that B additive in the alloys forms secondary phase CeCo₄B, which can improve high ratio discharge property. Special software MASTE was used to optimize the composition. It is found when the addition of B is 0.1 in atom ratio the comprehensive properties are the best. A new alloy composition Mm (NiCoMnAlB)_5.08 was predicted, and the electrochemical properties were proved by experiment.