In this paper, a novel, to the best of our knowledge, method is proposed to design a freeform off-axis three-mirror reflective imaging system. A special algorithm is demonstrated to calculate the data points on the unknown freeform surface using the rays from the pupil of the center field. Then the three-dimensional shape of the freeform surface is solved by these characteristic points, which serves as a good starting point for further optimization. The benefit of this design method is demonstrated by designing a freeform off-axis three mirror imaging system with high performance. The final system operates at F/2 with an entrance pupil diameter of 400 mm and a field of view of 2.4◦ × 2.4◦. The modulation transfer function (MTF) value of the system reaches 0.6 at 100 lp/mm or higher at all fields of view. © 2023 Optica Publishing Group.