For cooled 640×512 mid-wave infrared focal plane array, a compact dual field-of-view (FOV) optical system was designed. The parameters of the optical system were 4°×3.2°/12°×9.6° FOV, 4 F/number, and 100% cold shield efficiency. To reduce the overall length, the system used direct imaging model and a single negative lens moving along the axis to switch FOV. The system had 5 lenses including 2 aspheric surfaces. Based on analysis of chromatic and thermal properties of common MWIR materials to select lens materials and assign focal power, the system realized aberrations correction and passive athermalization. The image quality of prototype was good and its image plane was stable between -40°C~60°C.