The continuous zooming optical system has become more and more important for an infrared system in target searching and detecting.One method is changing two or more lens's location in order to change focal length in optical system.Based on the optic design features of medium wave infrared system the guide mechanism and cam mechanism of the continuous zooming mechanism are discussed in detail.The key points and result of the design are given.The test result indicates that the mechanism can operate smoothly and reliably with higher precision，and the image quality of optical system is fine.