The traditional optical compensated continuous zoom medium wave infrared optical system is improved by means of the combination of the zoom group, compensation group, and post-fixed group. The optical system with super-long focal length, super-large field of view, large zoom ratio, and small envelope is obtained. The mathematical model is deduced. Based on this model, a continuous zoom optical system with focal length of 11.56-982.6 mm and zoom ratio of 85× is designed by using 640×512 refrigerated medium wave detector. The application band of the optical system is 3.6-4.9 μm, the F number is 5.5,and the cold screen efficiency is 100%. The design results show that the envelope of the optical system is 333.5 mm×125 mm (local diameter of 177.5 mm)×80 mm (local diameter of 180 mm). The structure is compact, the zoom compensation curve is smooth, and the image plane is stable in zoom process. The image quality of the optical system is evaluated from the aspects of point diagram, optical transfer function, and distortion. The results show that the optical system has excellent image quality and high resolution, and meets the requirements of the thermal imager.