To realize super miniature, long focal length and super wide field-of-view, a highly integrated infrared optical system with four fields of view is proposed and designed. The system adopts a double optical path structure including a super narrow field-of-view optical system and a narrow field-of-view/middle field-of-view/wide field-of-view optical system. The two optical paths share a relay group. The super narrow field-of-view optical system is folded four times in three dimensions. The narrow field-of-view/middle field-of-view/wide field-of-view optical path is folded twice. Through above six folds, the size of the optical system including two optical paths is reduced effectively. Its shape envelope is in the range of 242 mm×150 mm × 85 mm (local 125 mm). Thus, the optical system is highly integrated. This double optical path optical system has four fields of view including a super narrow field-of-view, a narrow field-of-view, a middle field-of-view and a wide field-of-view. Among them, the super narrow field-of-view has a focal length of 688 mm and a field-of-view of 0.8 degree. The large field-of-view has a focal length of 13.19 mm and a field-of-view of 40 degree, in which long focal length, super large field-of-view and zoom ratio of 50 are realized. The super narrow field-of-view optical system merely uses five lenses. It has the features of high transmittance and passive optical athermalization design. The whole dual optical path optical system has compact structure and small size and realizes its super miniaturization. The design results show that the image quality of the optical system is good, which meets the requirements of thermal imagers.