Using high-purity tungsten powder and amorphous boron powder as raw materials, high-purity W₂B alloy powder was efficiently synthesized at low temperatures by mechanical activation combined with reactive synthesis. The effects of mechanical activation time on the morphology, particle size distribution, and specific surface area of the powders were investigated, and the relationship among phase composition, synthesis temperature, and reaction mechanism was elucidated. The results indicate that mechanical activation can effectively refine the particles, and the surface area and dislocation density of the powder increase with the prolongation of the mechanical activation time. The content of the W₂B phase in the reaction-synthesized powder increases as the mechanical activation time increases. After 20 h of mechanical activation, the true density of the reaction-synthesized powder reaches 17.01 g/cm³, with the W₂B phase content of 96wt%. This powder contains 23wt% more W₂B phase compared to the powder without the mechanical activation reaction. During the reactive synthesis, the B atoms diffuse into the W matrix, resulting in the formation of the low-density WB phase. Mechanical activation introduces a significant number of dislocation defects, which creates a channel for atom diffusion and accelerates the transformation from the WB phase to the W₂B phase.