Abstract: The preparation of permanent magnetic materials using high-abundance mixed rare earths can not only effectively reduce the cost, but also promote the balanced utilization of rare earth resources. In this paper, high coercivity mixed rare-earth permanent magnets were prepared by grain boundary diffusion process, and the effects of heavy rare-earth Tb on the magnetic properties and microstructure of the mixed rare-earth permanent magnets were investigated. It was found that when the grain boundary diffusion heat treatment was 880 ℃ for 8 h, the magnets exhibited excellent magnetic properties, and the coercivity increased from 660.12 kA/m to 1 248.13 kA/m, and the remanent magnetization remained at 1.29 T. The diffusion of Tb elements leads to the formation of (Tb, RE)₂Fe₁₄B core-shell structure at the main-phase grain edges of the mixed rare-earth diffusion magnets, which not only enhances the magnetocrystalline anisotropy field at the main-phase grain edges, but also blocks the magnetic coupling between the grains, inhibits the growth of reverse domain nuclei, and improves the coercivity of the magnets.