Recent advances of uncooled detector technology especially the development of uncooled micro-bolometer array hold promise for us to develop low-cost and compact MWIR earth observation imager. For comparative lower radiometric performance of uncooled focal plane array, fast speed optical system operating in large spectral bands is compatible. In addition, in order to exhibit advantages over imagers based on cooled detector technology, the optical system should be as compact as possible which means fewer elements, smaller size and light weight. In this article, a high speed optical design meeting these requirement is provided with 100mm focal length, F/1 F number,±2.5°field of view woking in 3-5um wave band. The fast speed MWIR imager has properties mentioned as follows: First, the optical system utilizes a hybrid system including refractive and diffractive elements. Second, the optical system realizes athermalization in simple passive way through distributing power among the refractive elements. It can work under typical temperature scope from -20°C to 60°C for typical space application. Third, Because of high speed aperture, the design makes use of aspheric surface to correct spherical aberration and spherochromatism .Finally, we use Ge and Si material. instead of expensive ZnS material.