La0.8GdxMg0.2Ni3.15Co0.25Al0.1 (x = 0–0.4) hydrogen storage alloys are prepared by induction melting followed by annealing treatment at 1173 K for 8 h. The effects of non-stoichiometry on phase structure and electrochemical properties with Gd addition are investigated systematically. The results show that Gd addition exerts significant effects on the phase component of the alloys. A appropriate amount of Gd addition (x = 0.1) can increase the abundance of (La,Mg)2Ni7-type phase while that of (La,Mg)5Ni19-type phase decreases. The further increase of Gd content (x ≥ 0.2) results in appearance of (La,Mg)Ni3-type, (La,Mg)7Ni16-type and La2MgNi2-type phases. Electrochemical tests reveal that the maximum discharge capacity and cycle life of the alloy electrodes can benefit from the right amount of Gd addition (x = 0.1), both of which are deteriorative with further increasing Gd content. However, the excess amount of Gd (x ≥ 0.2) jeopardizes the high rate dischargeability of the alloy electrodes obviously owing to the increase of charge transfer resistance on alloy surface.