AZ91-La-Yb magnesium alloy as anode of seawater batteries was prepared by combining mechanical alloying with spark plasma sintering processes. The effects of rare earth La-Yb doping on the microstructure and electrochemical behavior of AZ91 anode were studied. The results show that the AZ91-La-Yb alloy prepared by mechanical alloying-spark plasma sintering processes consists of equiaxed grains. On the one hand, La-Yb doping results in the formation of micron-scale (0.5–2 μm) RE-rich phase that are uniformly distributed at grain boundaries. This phase is mainly composed of rare earth metals (RE=La, Yb) and Mg(RE) solid solution. On the other hand, the plastic deformation caused by discharge plasma sintering and the doping effect of rare earth elements La-Yb significantly improve the morphology of β-Mg₁₇Al₁₂ phase, transforming from a coarse network structure to a slender elongated shape. The combination of uniform distribution of nearly micron-scale RE-rich phase and the smaller β phase promotes the uniform dissolution of magnesium alloys and effectively alleviates localized corrosion of magnesium alloys. Compared to the AZ91 anode magnesium alloy, the AZ91-La-Yb alloy doped with rare earth La-Yb exhibits more stable discharge voltage and excellent discharge performance. At a current density of 20 mA/cm², its specific capacity can reach 1068 mAh/g, and the anode utilization efficiency is 50.4%.