Abstract: In the process of ion beam polishing, the determination of dwell time is very crucial. Usually the ideal Gauss function is used to approximate the practical processing function. If the practical processing function is used to simulate processing, the result of processing is not satis-facotry. A synthesis algorithm of coefficient method and elimination method is proposed to improve the accuracy of mirror face when the practical processing function is adopted to simulate processing. Firstly, it uses the coefficient method iteratively to get a smooth mirror face, and then uses the elimination method to improve the accuracy. This algorithm runs faster, and works out a smooth mirror face with high accuracy. The synthesis algorithm is simulated, the results between the ideal Gauss function and the practical processing function are compared, as well as the mirror faces obtained by elimination method and synthesis method. The simulation results show that, compared with the 83.36 nm PV value of the practical processing function without the synthesis algorithm, the one with the synthesis algorithm is decreased to 46.92 nm, and the accuracy of mirror face is improved.