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 and combination reactions. The effects of mechanical activation time on the morphology, particle size distribution, and specific surface area of the powders were investigated, and the relationship between phase composition, synthesis temperature, and reaction mechanism was elucidated. The results indicated that mechanical activation could effectively refine the particles, and the surface area and dislocation density of the powder increased as the mechanical activation time lengthened. The content of the W2B phase in the reaction-synthesized powder increased as the mechanical activation time increased. After 20 hours of mechanical activation, the true density of the reaction-synthesized powder reached 17.01 g/cm3, with the W2B phase content of 96 wt%. The powder synthesized by thatcontained 23 wt% more W2B phase compared to the powder without the mechanical activation reaction. During the synthesis reaction, the B atoms diffused into the W matrix, resulting in the formation of the low-density WB phase. Mechanical activation introduced a significant number of dislocation defects, which created a channel for atoms diffusion and accelerated the transformation of the WB phase to the W2B phase.