For cooled 320×240 detector with staring focal plane array,a novel middle infrared athermal optical system is presented. The system is composed of 5 spherical lenses. The materials of lenses are silicon and germanium. The optical parameters and modulation transfer function (MTF) are investigated. The system has the diffraction limited image quality and stable image plane from -30 oC to 70 oC. The characteristic parameters of the system are as follows: f/number of 4,cold shield efficiency of 100%,spectrum region of 3.7-4.8 μm and transmissivity of 80%. The system has the merits of simple structure,low price,and it is easy to machining.