The invention discloses a Sm-Co based permanent-magnet thin film and a preparation method thereof, and relates to the technical field of Sm-Co based permanent-magnet thin films. The Sm-Co based permanent-magnet thin film comprises a substrate and a Sm-Co thin film layer, wherein the Sm-Co thin film layer is arranged on a surface of the substrate, a buffer layer is arranged between the substrate and the Sm-Co thin film layer and has a four-layer structure, one layer is a Cu thin film layer arranged on the surface of the substrate, a W-Cu thin film is arranged on a surface of the Cu thin film layer, a Ni-W thin film layer is arranged on a surface of the W-Cu thin film layer, and a Ni thin film layer is arranged on a surface of the Ni-W thin film layer. By using a magnetron sputtering method, the obtained Sm-Co based permanent-magnet thin film has relatively high film-based binding force, no crack appears after high-temperature processing on the Sm-Co based permanent-magnet thin film, and the Sm-Co based permanent-magnet thin film has relatively high magnetic performance.