Follow the traditional evaluation method for the absolute testing of three optic flats, the result of the deviation can be evaluated by the deviation on two perpendicular diameters, its evaluation method is different from the true surface graph exactly. This text introduces a rotation method for the absolute testing of three optic flats, which is based on the photoelectric detecting technique, laser interference technique and image manipulation technique, it can solve the deviation on N diameters of each optic flat and enhance the accuracy.