A method for manufacturing a diamond film is provided. The material with a low thermal decomposition point is used as a substrate. A buffer layer is formed on the substrate by coating or deposition, and then a diamond film is coated thereon, fitting the shape of the required diamond film. With the buffer layer, the coating or deposition uniformity of the diamond film is improved, and the problems such as thermal stress cracking and assembly damage are solved as well. During a subsequent process of forming the diamond film, the substrate is thermally decomposed due to a high temperature, such that the problems such as stripping and die loss are overcome.