In order to evaluate the effect of stoichiometric ratio upon electrochemical properties of AB5-type hydrogen storage alloys, a series of alloys Mm0.8La0.2(Ni4.0Mn0.5Al0.3Co0.37Fe0.13)a (a=0.90-1.08) were prepared and the electrochemical properties were tested. Mm0.8La0.2Ni4.0Mn0.5Al0.3Co0.37Fe0.13 was one of some new low cobalt AB5-type hydrogen storage alloys which had been researched to have good electrochemical properties. The results show that the stoichiometric ratio has great effects on the electrochemical properties of alloys. And the effects were investigated in detail. When stoichiometric ratio x＜5.3, the activation performances of alloys are all good. But when stoichiometric ratio x＞5.3, the activation performances are decreased evidently. When stoichiometric ratio x＜5.3, the discharge capacities of alloys are linearly increased with the increase of stoichiometric ratio. When stoichiometric ratio x＞5.3, the discharge capacities of hydrogen storage alloys decrease linearly rapidly by the raise of stoichiometric ratio. Overstoichiometry is good to the cycle lives of alloys. When stoichiometric ratio is between 4.8 and 5.4, voltage platforms of alloys tend to increase linearly with the increasing of x. When x＞5.4, the increase of stoichiometric ratio leads to the decrease of voltage platform of alloys.