For the purpose of optimizing the polishing process of ultra-precision optics, the optimal motion for bonnet tool used in conformal polishing (CP) is investigated. Firstly, the removal mechanism in CP was studied and analyzed, based on which the conformal polishing conditions (CPC) and the conclusion that sub-steps precession (S-s-P) is the optimal motion were obtained; After that, corresponding simulations and experiments were conducted, and the results revealed that (1) the ability for CP of continuous precession (Con-P) was not stable, and only when the round distance S and the total polishing length are sufficiently tiny and large, meanwhile, S / vfx = 2*π/ωA is satisfied, the conformal polishing performance of Con-P is good. However, these requirements would limit its application in engineering; (2) Both the incline polishing (IP) and S-s-P models have good performance on CP, and their abilities for CP were not affected by the feeding speed. However, since the texture of the polished area of IP is not so good as S-s- P, the later is the most favourite model in CP. All the results have proven the correctness of the CPC and the theoretical conclusion, which is useful for the engineering application of S-s-P model of bonnet tool.