For a cool 320 pixel×256 pixel detector with staring focal plane array, an infrared continuous zoom optical system using mechanical compensation and folding beam path for middle wavelength infrared (MWIR) is designed. The system is composed of a zoom system and a secondary imaging system, including seven lenses, two reflectors and three even aspheres. The computing process is introduced to obtain structural parameters of the optical system, and then optimization is applied by Zemax. Image evaluation, cam curve and cold reflection are listed. The design results show that the system which works at 3.7~4.8 μm has achieved the zoom of 18~360 mm, and satisfied 100% cold shield efficiency. The modulation transfer function is always bigger than 0.5 at the spatial frequency of 16 lp/mm. The system can be used for airborne optical-electronic detection.