In order to solve the problems when the R-C reflect IR system and the stability of the second mirror is installed, a compact integer structure is presented. The front and rear surface of lens are machined. Then inner reflect films are coated on two surfaces. At last the two mirrors are assembled in one lens which makes the installation of the two mirrors easier and the structure of the second mirror more stable. Reflection and antireflection films are coated on different surfaces respectively. Then two reflective surfaces and two transmission surfaces are produced on single lens. The entire optical system is integrated on a lens. It can not only effectively reduce system complexity and the difficulty of alignment, but also improve the stability and reliability of the system. A design of medium wave infrared (MWIR) compact optical system is presented. It has good imaging capability with modulation transfer function (MTF) of all field of view more than 0.55 which is very close to the diffraction limitation. The optics system realizes athermal between -40 ℃~60 ℃by binary diffractive optical element. Finally a real lens is manufactured by a single point diamond turning machine and imaging experiments is carried out at different temperatures. The results show that the optical system performance well without defocus. The system meets the actual processing and application requirements.