In order to realize determined fabrication by the Magnetorheological Finishing(MRF), the coordinate origin of removal function was calibrated according to the lowest point of polishing wheel. The generation of removal function and the distribution of removing rate were analyzed. Then the coordinate relationship between the lowest point of polishing wheel and the testing probe was deduced by using a standard cylinder, realizing the precise alignment of optical elements. The removal function was tested on special points of the optics, and the coordinate origin of the removal function derived from the lowest point of the polishing pad was calibrated. Furthermore the calibrating error was analyzed clearly. The calibration experiment was performed on the polishing wheel with a diameter of 360 mm, where a circle plane mirror was fabricated with aqueous magnetorheological fluid made of diamond polishing powder. The precision of calibration is up to 0.030 mm. The result indicates that the proposed calibration method is simple and reliable, therefore can meet the demands of the MRF and establish a strong foundation for MRF in optical fabrications.