The grain boundary diffusion process (GBDP) has become an important technique in improving the coercivity and thermal stability of Dy-free sintered Nd-Fe-B magnets. The influence of Dy₇₀Al₁₀Ga₂₀ and (Pr₇₅Dy₂₅)₇₀Al₁₀Ga₂₀ alloys by the GBDP on sintered Nd-Fe-B magnets are investigated in this paper. After diffusing Dy₇₀Al₁₀Ga₂₀ and (Pr₇₅Dy₂₅)₇₀Al₁₀Ga₂₀ alloys, the coercivity (H_cj) of the magnets increased from 13.58 kOe to 20.10 kOe and 18.11 kOe, respectively. Meanwhile, the remanence of the magnets decreased slightly. The thermal stability of the diffused magnets was improved by the GBDP. The microstructure shows continuous Rare-earth-rich (RE-rich) grain boundary phases and (Dy, Pr/Nd)₂Fe₁₄B core-shell structures which contribute to improving the coercivity. Moreover, the Dy concentration on the surface of the (Pr₇₅Dy₂₅)₇₀Al₁₀Ga₂₀ diffused magnets decreased with the Pr substitution for the Dy element. The openness of the recoil loops for the (Pr₇₅Dy₂₅)₇₀Al₁₀Ga₂₀ diffused magnets is smaller than that of the original magnets and Dy₇₀Al₁₀Ga₂₀ diffused magnets. The results show that the (Pr₇₅Dy₂₅)₇₀Al₁₀Ga₂₀ alloys can effectively optimize the microstructure and improve the magnetic properties and thermal stability of the sintered Nd-Fe-B magnets.