Remote sensing mapping applications need optical system with long focal length, wide width, low distortion, small volume, and the optical system can be designed for integration on satellite platform. Through the structure selection, modified coaxial three mirror system is adopted to realize long focal length, wide width and low distortion at the same time. Because of the influence of the second obscuration and large field, the image quality of general aspheric optimization design cannot meet the requirements. Degrees of freedom are increased efficiently by the introduction of free-form surface. After optimization design, the optical design modulation transfer function (MTF) is greater than 0.418 at 72 lp/mm, and the maximum relative distortion is less than 0.00145%. Imaging quality is improved significantly. The free-form surface is tested and fine grinded by the application of computer-generated holography (CGH) technology, the root mean square (RMS) of remnant wavefront error is 0.007λ, and the peak to valley (PV) value is 0.027λ, remnant wavefront error meet the requirements of free-form surface irregularity tolerance. Laboratory static MTF is tested after processing and adjustment, the minimum static MTF is 0.225 at 72 lp/mm, the image quality meet the requirements of technical indicators.