The invention discloses a La-Mg-Ni series hydrogen storage alloy for a low-temperature nickel-hydrogen battery and a preparation method of the La-Mg-Ni series hydrogen storage alloy. The chemical component of the alloy is La<0.75-x>CexMg0.25Ni3Co0.5, wherein the subscripts in the chemical component are atomic ratios of corresponding constituents, and the value range of x is 0.09-0.11. The preparation method of the alloy comprises the following steps of raw material preparation; smelting and casting; alloy thin strip preparation; and grinding and screening. The alloy prepared by the method hashigh low-temperature electrochemical performance, and under the condition of 248K, at the quenching rate of 10 m.s<-1>, when charging and discharging are circulated 100 times, the discharge capacity of the alloy reaches the maximum value of 245.9 mAh.g<-1>. An alloy material prepared by the preparation method is particularly suitable for being used under the condition of zero DEG C or below, particularly under the condition of minus 25 DEG C and even under the condition of minus 30 DEG C.