To improve the surface quality of polyimide (PI) optics, a polishing method for polyimide membranes based on Reactive Ion Etching (RIE) was proposed. The working principle and experimental research for the proposed method were discussed in this study. Owing to the low surface tension and mobility of the PR fluid, surface defects on the PI surface were required to be filled by a PR coating. In addition, owing to the highly anisotropic and identical RIE rates for PR and PI, the smooth PR surface could be precisely transferred onto the PI surface. The polishing of PI membranes then was realized. Experimental results indicate that for a surface roughness with PV of 1.347 μm and RMS of 340 nm, the respective values can be reduced to 75 nm and 13 nm after double polishing. Furthermore, for a surface roughness with PV of 61 nm and RMS of 8 nm, the values can be reduced to 9 nm and 1 nm, respectively. Hence, the proposed polishing method based on RIE can efficiently improve the surface finish of PI membranes, which in turn can provide novel schemes for precision manufacturing of PI-based flexible polymer devices.