In order to improve the electrochemical cycle stability of La-Mg-Ni system (PuNi3-type) hydrogen storage alloy, a trace of Cr was added in the alloy. La2Mg(Ni0.85Co0.15)9Crx (x= 0, 0.1 and 0.2) hydrogen storage alloys were prepared by casting and rapid quenching. The electrochemical performances and microstructures of the as-cast and quenched alloys were determined and measured. The effects of Cr content and quenching rate on the microstructures and electrochemical properties of the alloys were investigated in detail. The obtained results show that the as-cast and quenched alloys are composed of the (La, Mg)Ni3 phase (PuNi3-type structure) and the LaNi5 phase as well as the LaNi2 phase. The rapid quenching does not have much influence on the phase compositions of the alloys, but changed the phase abundances of the alloys obviously. The addition of Cr enhances the cycle stability of the as-cast and quenched alloys, but decreases the discharge capacities of the alloys. The cycle lives of the alloys increase with the increase of the quenching rate. The as-cast and quenched alloys have an excellent activation performance.