Disclosed is a method for preparing a rare-earth permanent magnet. The method includes: preparing an R-T-B-based sintered magnet; applying a first mixture including a light rare-earth element onto the surface of the R-T-B-based sintered magnet and diffusing the first mixture under a vacuum atmosphere to prepare a light rare-earth permanent magnet having the light rare-earth element diffused into a grain boundary; and applying a second mixture including a heavy rare-earth element onto the surface of the light rare-earth permanent magnet and diffusing the second mixture into the grain-boundary under a vacuum atmosphere to prepare a rare-earth permanent magnet.