The traditional continuous zoom middle wave infrared optical system is improved by the separation optical compensation method. The zoom group and the compensation group are separated independently to realize zoom and compensation, and the middle wave infrared optical system satisfies the ultra long focal length (916.2 mm), the ultra large field of view (36°) and the miniaturized demand of continuous zoom optical system. In zoom, the zoom compensation curve is smooth, no turning point, and the image surface is stable. The envelope of the optical system is within the range of 335 mm×118 mm (local 169 mm)×100 mm (partial 168 mm), the system is compact. The image quality evaluation of the optical system indicates that the image quality is good, and it can meet the requirements of the thermal imager.