In order to meet the requirement of tank sight and improve the ability of joint transform correlator in target tracking and recognition, an infrared continuous refractive zoom optical system for 8~12 μm wave-band is designed for 1 inch infrared CCD detector. The zoom system uses mechanical compensation. The relative aperture is invariant during the zoom process. The F number of the system is 2, and the zoom ratio is 4∶1.The focal length from 50 mm to 200 mm could be changed continuously. Variable magnification curve and compensation curve are smooth. The range of tank sight searching targets was expanded. The optical system image quality is evaluated with ZEMAX optical design software. The results have shown when spatial frequency is 17lp/mm, the MTF curve in the whole range of focal length are all above 0.55 which is closed to the diffraction limited curve. The stability of the image plane is well. The result of this system meets the requirement of technical specification.