Usually, infrared optical system design and test in normal temperature conditions. In fact, infrared system work in the
larger temperature range. In this paper, for satisfying the special environmental tempreture requirements, an optically
passive athermal infrared optical system working in the 3~5 micron mid wavelength infrared band was designed. The
design principle, design results are described. In addition, the optical system was incorporated with an infrared focal
plane array forming an infrared camera. The thermal test of imaging quality of this camera is also presented,which
validated the athermalisation.The thermal testing method is simple, intuitive, high credibility. The result shows that the
IR optical system has good athermal performances in large working temperature range with simple configuration.