：A new effective combined design method for low obscuration ratio (OR) three-mirror imaging system is proposed, which involves the correction of varied aberrations, such as spherical aberration, coma, astigmatism, etc., as well as the control of the OR introduced by the secondary mirror’s (SM) baffle effect at the optical design stage. The initial theoretical works include the derivation of the all-spherical system’s 3th-order aberration equations and the calculation of the baffles’ edge coordinates using paraxial ray-tracing. Next,the aberration equations are solved with the boundary conditions of suppressing stray lights to determine the focal powers and first-order parameters of each mirror. And then the system’s initial configuration is obtained as the starting point. During the optimization, conic aspheric surfaces are employed to correct aberrations with the consideration of decreasing the manufacture cost and fabrication difficulties. Meanwhile, the relationship between the SM’s baffle size and the baffles’ edge coordinates based on real ray-tracing is built to minimize the obscuration. Finally, a three- aspheric-mirror system with low OR and high imaging performance is achieved. An infrared-middle-wave system design of % OR is shown and the design result shows that the system’s limited spatial frequency is 33lp/mm and the MTFs of each field-of-view approach the diffraction limits. The stray-light analysis results utilizing FRED software verify the stray light suppression effectiveness of the baffles design of the proposed method.