Experiments is designed to research the instability of fluid jet polishing system. The results show that the material removal shape is dissymmetrical, and the material removal ratio is fluctuating with different polishing regions even on the same polishing condition. Then the influence factors of the instability is analyzed, such as pressure fluctuation, abrasive particles deposition, turbulent intensity and so on. The fluctuation of those error factors are acquired from experiments and simulation. Based on these data, the uncertainty is established for material removal ratio, showing that the theoretical uncertainty accords with the experiments′ error. Polishing precision can be improved by error factors′ compensation.