For the cooled 640×512 detector with staring focal plane array, a mid-wave infrared (MWIR) continuous zoom optical system with 30× zoom range was presented .The design idea of the continuous zoom system consisting of the zoom objective lens group and the relay lens group without the rear fixed group was introduced. The image quality of the system at three positions of short effective focal length(EFL), middle EFL and long EFL is given; moreover,the amount of defocus and the distortion of image quality reflected in the full focal length range was analyzed. The experiment results indicate that the optical system with 3.7 μm～4.8 μm working waveband can realize 18 mm ～540 mm continuous zoom,the defocus amount of the full focal length is less than the depth of focus, the maximum distortion of the long EFL area is close to 0, and that of the middle EFL is less than 3%. It has the advantages of large zoom range, simple structure, short zoom path, and smooth zoom locus,which can be used for infrared optical observation and tracking system.