The invention provides a diffusion method for preparation of high-performance NdFeB magnetic powder. NdFeB fine powder and heavy rare earth metal Dy blocks are placed in different treatment chambers in a heat treatment furnace respectively; the inside of the heat treatment furnace is vacuumized, then the whole heat treatment furnace is heated, the temperature in the treatment chamber reaches 600-800 DEG C, and the temperature is kept for 1-30 hours, so that the heavy rare earth metal Dy blocks form Dy vapor; at the time of heating, a furnace body of the heat treatment furnace is rotated, the Dy vapor thus makes full contact with the NdFeB fine powder and diffuses into the surface of the NdFeB fine powder, and thus a uniform (Nd, Dy) 2Fe14B shell layer is obtained. A sintered Nd-Fe-B magnetprepared from the Nd-Fe-B magnetic powder has the characteristics of being high in coercivity and high in thermal stability.