As the method to get the removal function by traditional Ion Beam Figuring (IBF) has complicated operation and higher costs, this paper proposes a new method to calculate the removal function. The method uses the Faraday cup scan to measure and calibrate the temporal distribution of ion beams and to obtain the different working parameters of ion sources which corresponds to removal functions. Firstly, the material removal theory of IBF was studied based on the Sigmund sputtering theory, and the influence of ion beam distribution on the removal function was researched. A simplified IBF removal function model was proposed. Then, an experiment was conducted to obtain the relationship between the temporal distribution of ion beam and the related parameter of removal function, and the different removal functions of various working parameters were calculated with the combination of the basic removal experiment and Faraday cup scan results. Experiments were performed on silicon and fused silica, and the experimental results show that the peak value of the removal function for the same material is proportional to the that of the scan result. A mirror with a Φ800 mm silicon flat modification layer was polished, obtained the Root Mean Square(RMS) of surface error is 57866 nm. After polishing for two times, the RMS is 11.837 nm, and the convergence rate is up to 4.89. These results meet the requirements of optical fabrication for polishing accuracy and improve the determining efficiency for removal functions.