Germanium and silicon monoxide are two kinds of optical thin film materials used frequently in medium-wave infrared band. Study on their optical properties is the key to design and fabricate medium-band thin film. Germanium and silicon monoxide are selected as high and low refractive index materials respectively and the substrate is synthetic sapphire. Triple cavity band-pass filter of which the center wavelength is 3.5 μm, relative bandwidth is 4.8%, and transmittance of pass-band is about 80%, was designed and fabricated. Spectrum measurement and environment reliability experiment show that the infrared band-pass filter is competent for engineering application.