Abstract: Zoom system can quickly find and search for the target in a wide range,while more detailed information of the object can be observed in a small range of view.Therefore the application and demand of infrared zoom system in defense, industrial, military and other fields have been growing.We introduced the theory and methods for designing continuous zoom lens,and completed the design of a continuous zoom infrared lens with focal length range of 30mm~110mm, working wavelength of 8 μm~12 μm and F number of 1.0,which was used in conjunction with a 320×240 pixel uncooled detector with the pixel size of 25 μm×25 μm.The total length of the optical system is less than 230 mm, the entire system consists of 7 lenses, and only uses the germanium material, all surfaces are spherical. Finally,we conducted the tolerance analysis the infrared zoom system, and obtained a set of loose machining tolerance.The results show that in the short focus of 30 mm, the modulation transfer function (MTF) value of system is greater than 0.3 at 20 lp/mm,while it is greater than 0.317 4 at 20 lp/mm in the long focus of 110 mm.