A compact integer structure with two mirrors was presented to solve the problems that the R-C reflect IR system is difficult to be installed into a seeker and the second mirror support is not stable in missile flight. The primary and secondary mirrors were integrated the both surfaces of a lens. Then, the front and rear surfaces of lens were machined and the inner reflect films were coated on two surfaces. The two mirrors were assembled in one lens to allow the installation of the two mirrors to be easier and the structure of the second mirror to be more stable. A compact relay imaging optical system for middle infrared was designed. It has good imaging capability and its Modulation Transfer Function(MTF) for all field of view is more than 0.6(very close to the diffraction limitation). The optics system can implement the athermal between -40~60 ℃ by a Diffractive Optical Element(DOE). Finally, a Ment-Karol simulation was performed to analyze the tolerance and some ways to resist the stray light of this system was given. The design satisfies the need of machining and application.