In order to break through the limitations of super-miniaturization, super-long focal length and super-wide field of view on optical system, a miniaturized four field of view medium-wave infrared optical system is proposed and designed. The optical system adopts double optical path structure, including super-small field of view optical path and small field of view/medium field of view/wide field of view optical path. Two optical paths share a relay group. The super-small field of view optical path is stereo-folded four times, and the small/medium/wide field of view optical path is folded twice, totally six times. The shape size of the whole dual paths optical system is effectively constrained, and the envelope is in the range of 250mm×155mm×90mm (local 140 mm), the optical system has high integration; the dual-path optical system includes four fields of view: super-small field of view, small field of view, medium field of view and wide field of view. The super-small field of view focal length is 733mm, the field of view is 0.75 degrees, the wide field of view focal length is 10.78 mm, and the field of view is 48 degrees, which realizes both super-long focal length and super-wide field of view with a zoom ratio of 68× and the super-small field of view optical system only adopts with five lenses, the transmittance is high, and the whole optical system with two optical paths has compact structure and small volume, thus realizing the miniaturization. The design and test results show that the optical system has good image quality and meets the requirements of high performance thermal imager.