An The fundamental power input coupler is a component of the free electron laser device, which transmits microwave power to the superconducting cavity. In order to improve the transmission performance, the coupler needs to be coated with copper film. In this paper, the coupler was plated with copper film by electroless plating and electroplating, respectively. Though the linear scanning, roughness, X-ray diffraction and residual resistivity ratio, the performance changes of the copper films prepared by these two copper plating methods at room temperature (25 ℃) and after vacuum annealing temperatures (200 ℃, 400 ℃, 600 ℃ and 910 ℃) were investigated. Furthermore, the copper plating technology for the inner surface of bellows of the coupler was determined, and this process was applied to the 1.3 GHz fundamental power input coupler. The results show that the bonding force between the copper film and the inner surface of the coupler and the efficiency of microwave energy transmission can meet the requirements of practical application.