Abstract: The magnetic properties of the sintered Nd–Fe–B magnets treated by the grain boundary diffusion of the sputtered Dy₇₀Cu₁₅Ga₁₅ alloys via annealing were investigated. The microstructure and corrosion resistance behavior of the magnets in the electrochemical environment after the grain boundary diffusion treatment were also characterized. The results show that the grain boundary diffusion treatment can significantly improve the coercivity of the magnets, which increases from 15.86 kOe to 19.46 kOe with an improvement rate of 22.7%, while the residual magnetism and the maximum magnetic energy product remain basically unchanged. Dy mainly diffuses into the grain boundary phases of the magnets and optimizes the distribution of the grain boundary phases. Dy existing in the defect zone of the main phase grain boundary replaces Nd in the main phases to form (Nd,Dy)₂Fe₁₄B phases, enhancing the anisotropic field of defect layer and suppressing the nucleation of demagnetized domains. The substituted Nd element enters the grain boundary region and thus increases the proportion of non-magnetic phases, effectively isolating the adjacent main phases and weakening the magnetic coupling. The eutectic diffusion of CuGa alloys with low melting point opens up the diffusion channels, which improves the diffusion efficiency of Dy, and optimizes the grain boundary structure. After the grain boundary diffusion, the X-ray diffraction peaks of the main phases shift towards to the larger angle, indicating the better grain orientation. Furthermore, the Dy-rich rare earth phases formed at the grain boundary are more stable with the higher corrosion potential, showing the optimized intergranular structure and the improved corrosion resistance for the sintered Nd–Fe–B magnets after the grain boundary diffusion.