Large imaging plane middle wave infrared continuous zoom optical systems were studied. A large imaging plane mechanical positive group compensated continuous zoom optical system was designed. The system had its operation waveband of 3.7 to 4.8 m, the focal length of 50 to 580 mm, the F number of 4.5 and the image plane diameter of 24.6 mm. It was suitable for the newly launched 1280×1024 pixel, 15 m pitch middle wave infrared cooled detectors. The system could keep a large field of view and effectively improve the ability of airborne electro-optical systems to search and identify targets while achieving long focal length and high resolution. The design result showed that the system had high imaging quality and its modulated transfer function value is close to 0.2 at the spatial frequency of 30 lp/mm.