According to the characteristics of chalcogenide glass with high thermal stability, perfect spectrum transmittance and moldable property, a design method for long-wave infrared optical systems only with chalcogenide glasses is proposed. The design process of passive optical athermalization and passive mechanical athermalization is analyzed. The initial optical power contribution is calculated based on the theory of passive optical athermalization. An infrared athermalized optical system is designed. The focal length is 90 mm, the Fnumber is 0.9. This system is compatible with uncooled long-wave infrared (8~14 μm) focal plane array which has a format of 640 pixel × 512 pixel and a pixel pitch of 17 μm. It can adapt within the temperature range of -40 ℃~50 ℃ , the modulation transfer function (MTF) of full field of view is close to the diffraction limit through the entire temperature range. The results show that the passive athermalization can be realized by matching chalcogenide glasses and using passive mechanical compensation, and the low-cost infrared imaging optical system can be achieved.