The freeform surface is a non-rotationally symmetrical surface with strong asymmetric aberration correction capability. In order to meet the requirements of the space remote sensing system for large range and high resolution imaging， an off-axis three-mirror reflective system with large field of view and long focal length is designed by using freeform surfaces. Firstly， the initial configuration with conic surfaces is obtained directly based on the vector aberration theory. The obscuration of the initial configuration is avoided by using the decenter and tilt of the mirrors. Then， freeform surfaces are used to correct the aberration of the initial configuration. The final freeform off-axis three-mirror reflective system is obtained by optimizing the initial configuration. The field of view of the designed system is 16° × 1°， the focal length is 2 000 mm， and the entrance pupil diameter is 235 mm. The modulation transfer function curves are close to the diffraction limit， and the maximum relative distortion is 0.37%. The optical system has good imaging quality within the tolerance ranges.