La0.5Nd0.05Sm0.30Mg0.15Ni3.5 alloy was prepared by induction melting followed by annealing treatments. The effect of annealing time on the microstructure and electrochemical performances were investigated. Rietveld analyses results show that all the alloys mainly consist of major A2B7 and A5B19 phases and residual AB5 phase. Pr5Co19-type phase disappears with annealing treatment time prolonged to 9 h. The A2B7 phase abundance first increases and then decreases with the increasing annealing time from 6 to 12 h while A5B19 and AB5 phase abundances show the reverse trend. Moreover, a-axis parameter, c-axis and unit cell volume of the phases remain almost unchanged with prolonging the holding time. The activation properties of the alloy electrodes have not been affected by annealing time. The maximum discharge capacity of alloy electrodes monotonically increase from 326.5 mAh g−1 (t = 6 h) to 362.2 mAh g−1 (t = 12 h). HRD2000 decreases from 63.8% (x = 0) to 55.5% (x = 0.20), which is determined by hydrogen diffusion coefficient. The cycling capacity retention rate at the 200th cycle monotonically increases from 67.0% (x = 0) to 70.6% (x = 0.30), which should be ascribed to the improvement of corrosion resistance of alloy electrodes in the charging/discharging cycle.