Based on the theory of optical thin film, the theoretical design and optimization, actual production, and finished product testing methods of 3.7-4.8 m band-pass filter are introduced. In view of the problems of film properties and layers matching, Ge and SiO are respectively selected as high and low refractive index materials, and alumina is used as the substrate. The basic membrane system of the filter film is determined, and the membrane system is designed and optimized by using the Filmaster software. In the film evaporation process, a suitable plating process is selected according to the materials. The band-pass filter film with excellent reliability and spectral characteristics is developed by temperature control and ion assistation, and its spectral characteristics and film quality are tested. The process parameters are modified according to the design goals, and the feasible process is finally determined to develop a 3.7-4.8 m band-pass filter conforming to the design requirements for optical performance.