A method for calculating the initial structure of off-axis optical system with three mirrors based on aberration theory is introduced, and a three-mirror off-axis optical system is designed with this method, which is adaptable for space remote sensing system.The system has a wave band of7.7~10.3 μm, effective mirror length of 1000 mm, F-number of 2, and the full FOV is 1°.The detailed optimization process is presented.It shows that the system has the advantages of long focal length, large diameter, high image quality and compact structure, which can meet the requirements of the space remote sensing system.