Through the study on the structural features and aberration characteristics of the off-axis three mirror optical system, a freeform surface is used in the appropriate location of the system. A broad width off-axis three mirror optical system is designed, with focal length of 1000 mm, F-number of 7, field of view of 20°×10°. The modulation transfer function of this optical system is higher than 0.49 at spatial frequency of 50 lp/mm. Compared with the previous pushbroom imaging model, this optical system realizes frame imaging model, so the choice of detector is more extensive. Compared with other three mirror optical systems, the main advantage of this broad width off-axis three mirror optical system is that it can obtain a wider imaging field of view, enhancing the quality of remote sensor imaging.