Abstract: The infrared multi-band optical system can track the band information stretching from mid-wave infrared to long-wave infrared ,which can greatly improve the information acquisition capability. A infrared multi-band optical system composed of 4 spherical lenses was designed based on the compact principle. It could image clearly at 4.4 m~8.8 m continuously , the F# was 2.68 which strictly matched with the cold light bar so that the cold light bar effect reached 100% . The system used the passive athermalization method to get rid of temperature compensation problem and finally realized athermalization for continuous bands through selection of lens materials and formula derivation. The tube was made of titanium alloy,the lens was made of ZnSe,Germanium and ZnS materials.The modulation transfer funtion(MTF) at 40 ℃ to 60 ℃ was given, as well as the distortion over every wavebands.The result shows that the design of the system structure is relatively simple, which satisfies the requirements of a standard infrared thermal imager.