An interferometric method for the absolute testing of optical flats, three- flat testing, is studied. This method needs three flats, which are combined in pairs in different positions, and measurements yield a vertical profile of data and a horizontal data profile along the diameter of the reference flat. According to Matlab and VC mixed programming, three- flat test of mutual inspection procedures is designed to quickly calculate experimental data with the help of an interferometer. Experiments are done to achieve high accuracy flatness measurement, and the calculation error is 3% relative to the Zygo software.