Abstract: The medium infrared band is widely used as an observation channel in applications such as space flight, meteorology and remote sensing. High quality medium bandpass infrared filters are often adopted in optical systems to improve signal-to-noise ratio. Relative bandwidth is a key specification for the medium bandpass infrared filter, which depends on the film structure and its design. Several film structures such as double cut-off structure, F-P structure, multi-half-wave structure are given, and their bandwidth estimation formulas are concluded. The relative bandwidths of the multi-half-wave structures were estimated and analyzed. The results indicate that half-wave structure is the most feasible solution. Relative bandwidth adjustment methods are put forward, such as using interval layer material, selecting optical thickness, using equivalent film, adjusting film refractive index. A medium infrared filter is designed with the suggested relative bandwidth adjustment method. The coating result shows that the relative bandwidth specification of the film meets the design requirement.