The study proposes a 30× continuous zoom lens for 640×512 cooled mid-wave infrared focal plane detector array. Furthermore, we propose an optical system design that separates the lens from the front fixed group as a two-speed moving group, and combines the two-speed moving group, variable magnification group, compensation group, and rear fixed group into a composite continuous zoom system. To realize the above model, we design a cooling-type medium-wave infrared continuous zoom optical system based on the theory of diffractive optics. It has a working band of 3.7--4.8 μm, F-number of 4, continuously variable focal length of 12--360 mm, and total optical length of only 160 mm. We optimize the image quality and cam curves of the system at 6 focal lengths. The system offers advantages of a large variable ratio, miniaturized structure, excellent image quality,and smooth cam curve, which meets the requirements of infrared thermal imagers.