8-12 μm infrared (IR) hybrid refractive/diffractive continuous zoom lenses is designed for the needs of target detecting/tracking system. The defects of conventional systems,such as being insufficient for achieving long focal length,large relative aperture,big zoom ratio,high imaging quality and simple structure,are solved in this system,which has the properities of large relative aperture,F=0.9,zoom ratio 11×,discernment with long focal lengths for tanks at about 1500 m distance. The optical system composes of seven pieces of Germanium lens. To correct chromatic and off axis aberrations,the system applies diffractive optical element (DOE) and aspherical surface. The modulation transfer function (MTF) is above 0.64 in all focal lengths at the spatial frequency of 17 lp/mm,which approaches the diffraction limit. The energy concentration ratio is greater than 82% in the 17.5 μm radius of sensing pixel. It shows that the optical system has good imaging quality and meets the needs of general requirements for the target detecting/tracking system.