In needs of stray light suppressing and system volume compression, a compact system of off-axis three-mirror structure with secondary imaging and real exit pupil is designed.A method of constructing the initial structure of the off-axis three-mirror optical system with secondary imaging is given.The optical system is optimized with freeform surfaces based on the initial structure, and the off-axis three-mirror optical system with the focal length of 130 mm, F-number of 1.3, the linear FOV of 5°×0.1° is designed.The system imaging quality is close to the diffraction limit, the system has the characteristics of stray light suppressing with secondary imaging and real exit pupil, and the 100% cold stop efficiency can be realized theoretically.The system is compact with the super-little F-number of 1.3 and can be applied to such military fields as aviation and aerospace detection where there are high demands of stray light suppressing.