In order to analyze the effect of removal function on machining accuracy and the anti-intermediate frequency error performance of removal function in magnetorheological finishing, the error suppression ability of magnetorheological finishing was evaluated by using the smoothing spectrum method. The smooth spectrum method and the traditional power spectrum method were theoretically analyzed, simulated, and verified by the aspherical optical element. The suppression ability of the removal function on the surface error and the mid-spatial error of the aspherical optical element were analyzed by the smooth spectrum method and the traditional power spectrum method. The results show that error suppression ability of smoothing spectrum method is 0.22 mm^-1. The smoothing spectrum function can intuitively judge the suppression ability of the surface error and mid-spatial error of the removing function, and the variation rule of mid-spatial error can be clearly quantified and judged. This method provides a solution to the problem of error suppression evaluation in magnetorheological finishing.