Abstract: The Nd-Fe-B magnet was prepared by the grain boundary diffusion process in which TbH₂ power was evenly coated on the surface of Nd-Fe-B and then diffused at different temperatures in the ground boundary of Nd-Fe-B. The effects of the diffusion of TbH₂ power in the grain boundary of Nd-Fe-B magnet on the magnetic properties at room temperature and thermal stability at high temperature were investigated, and the mechanism of coercivity enhancement was analyzed. The study of magnetic properties at room temperature showed that the performance of the magnet reached the best after 890 ℃+490 ℃ process and its coercivity increased from 1 383 kA/m to 1 988 kA/m. The study of high-temperature magnetic performance showed that the temperature coefficient of coercivity |β| of the magnet at 200 ℃ was reduced by 0.032% /℃ and the irreversible loss of magnetic flux (hirr) was reduced by 21.47% compared with the original one, and the thermal stability of sintered Nd-Fe-B magnet was significantly increased. Through analysis, it can be concluded that the mechanism of coercivity enhancement of the magnet caused by the diffusion of TbH₂ powder in its grain boundary lies in that the epitaxial layer of Nd₂Fe₁₄B grain forms a (Tb, Nd)₂Fe₁₄B core-shell structure which enhances the magnetocrystalline anisotropy field and that the microstructure of the magnet is improved so that the magnetic exchange coupling between the grains is effectively avoided.