The report tells about the design of a middle wave infrared continuous-zoom imaging optical system. To capture the image in an «eye-type» infrared optical system a multi-element sensor is used. To increase the contrast of the thermal image the sensor is equipped with a cold aperture. The use of an optical system based on the continuous-zoom lens makes it possible to solve the problems of object detection and identification more effectively but requires taking into account some features of varifocal lenses. The optical system being analyzed in the report includes front zoom objective system and secondary imaging system. The front zoom optical system is composed of four groups and provides the focal length that is continuously changed with a large zoom ratio (15–20X) under the condition of a constant total length of the system. The secondary imaging system is designed for zoom system exit stop and sensor cold stop conjugation. The design technique represents analytical expressions and equations to obtain optical parameters of the front and secondary system components and the motion curves for movable groups.