To improve the Graphical Representation of Model Spaces (GRMS) index of optical elements, a new polishing method based on smoothing surface shape by fitting was proposed after the rules of the effects of surface shape precision and distribution on the wavefront gradient index were summarized. The fundamentals and workflow of this method were then presented. First, the surface shape discontinuity was determinedby scanning and computing the original surface shape data. Then, the new surface shape data used in guiding the processing were generated using the non-uniform rational B-spline fitting algorithm and the data between the neighbor discontinuities was reconstucted. Finally, appropriate processing parameters were established based on the generated surface shape data. An experimental study was conducted on K9 material plane mirrors of 610 × 440 mm, and results showed that the proposed method can reduce the GRMS from 11 nm/cm to less than 7.7 nm/cm in two polishing cycles while maintaining a high surface shape precision.