In order to achieve the high quality alignment of an imperfect imaging off-axis three mirror system, a design method of aberration compensation system placed in front of the focus of the three mirror system is proposed. Based on Gaussian optics, the relationship between the parameters of the compensation system and the spatial position of the image points is derived. In addition, the initial structure of the compensation system is designed according to the third-order aberration theory. An off-axis three mirror system with an effective aperture of 400 mm and an F number of 3.9 is adopted to test the proposed method and the feasibility of this method is verified. The proposed method can effectively improve the optimization efficiency and is suitable for various types of reflection systems. The final design results can achieve the high-quality alignment of an imperfect imaging off-axis three mirror system with a relatively small aperture, effectively reduce the assembly-alignment cost, and improve the assembly-alignment efficiency.