Mg2Ni–x mol% Mg3MnNi2 (x =0, 15, 30, 60, 100), the novel composite alloys employed for hydrogen storage electrode, have been successfully synthesized by a method combining electric resistance melting with isothermal evaporation casting process (IECP). X-ray diffraction (XRD) analysis results show that the composite alloys are composed of Mg2Ni phases and the new Mg3MnNi2 phases. It is found on the electrochemical studies that maximum discharge capacities of the composite alloys increase with the increasing content of the Mg3MnNi2 phase. The discharge capacity of the electrode alloy is effectively improved from 17mAhg−1 of the Mg2Ni alloy to 166mAhg−1 of the Mg3MnNi2 alloy. Among these alloys, the Mg3MnNi2 phase possesses a positive effect on the retardation of cycling capacity degradation rate of the electrode materials. Cyclic voltammetry (CV) results confirm that the increasing content of the Mg3MnNi2 phase effectively improves the reaction activity of the electrode alloys. Surface analyses indicate that the Mg3MnNi2 phase can enhance the anti-corrosive performance of the particle surface of these composite alloys.