A fast-switching wide-band infrared dual-field-of-view optical system has been developed to meet the needs of long-range, weak-target, all-day detection in space environments and to improve the detection capability of large-span temperature variations on the surfaces of different targets. The system adopts the composite design of mid-wave infrared and long-wave infrared, with spectral ranges of 3.7–4.8 μm and 7.7–9.7 μm, focal lengths of 161 mm/323 mm and F-number of 2.5. To adapt to a wide range of operating temperatures in space, the optical system is designed to be athermalised from −40 °C to +60 °C. Meanwhile, in order to improve the detection effect of targets at different distances and reduce the probability of losing fast-moving targets during the field-of-view switching process, the zoom group adopts a rotating solenoid-driven cut-in/cut-out zoom mode to realise the fast switching of large and small field-of-view. The design results show that the optical system has the characteristics of fast field-of-view switching, wide spectral band, athermalised design and good image quality, which meet the requirements of detecting space debris and other targets throughout the day.