Interferometers are used to test the surface shape data of optical component for further processing to provide guidance. Various spatial point on the tested optical component are given by interferogram analysis. The binary image is get from the raw data, then the edge of optical component is differentiated with a sobel exactly. A coefficient constraint condition is incorporated into the least squares method for geometric parameters of circles, and the location of the centre of the circle and the size of the radius will be calculated by the method. Various spatial points on the tested optical component are given by calculation method, and arithmetic average value of equal precision is provied. The coordinate of the interferogram can be established exactly.