Grain refinement is an effective way to improve the mechanical properties of Bi_0.5Sb_1.5Te₃ based alloys. However, the donor-like effect caused by the grain refinement during powder metallurgy severely degrades the thermoelectric properties of the material, which limits the application of Bi_0.5Sb_1.5Te₃-based alloys in micro thermoelectric devices. Therefore, in this study, focusing on p-type Bi_0.5Sb_1.5Te₃-base alloy, the impact of grinding and desorption atmosphere on donor-like effect and thermoelectric transport properties of sintered samples during powder metallurgy process were systematically studied through experiments combined with theoretical calculations. Surface defects $~\text{V}_{\text{Te}}^{\cdot \cdot }$ and ${{\text{{V}'''}}_{\text{Sb}}}$^， generated by the grinding process of Bi_0.5Sb_1.5Te₃-based alloy, reacts with the adsorbed O₂ during sintering process. This generates a large number of $\text{V}_{\text{Te}}^{\cdot \cdot }$ vacancies and free electrons, leading to a donor-like effect and a significant decrease in carrier concentration. The use of protective atmosphere (Ar atmosphere) during grinding to avoid exposure to air or O₂ desorption under protective atmosphere after grinding effectively suppresses the donor-like effect, maintaining high carrier concentration and electrical conductivity of the sample. Moreover, the performance is stable up to 473 K. The samples treated in air or sintered using powder placed in air exhibit a significant donor-like effect, and the carrier concentration decreases from 4.49×10¹⁹ cm^–3 to 3.21×10¹⁹ cm^–3. The maximum thermoelectric ZT of 1.03 is obtained at 402 K for the samples sintered with the powders treated under protective atmosphere, and the overall average ZT_ave reaches 0.92. This study provides a new avenue to regulate the donor-like effect of p-type polycrystalline Bi₂Te₃-based compounds and to optimize their thermoelectric properties.