Interferometric testing the vertex radius of curvature (ROC) and conic constant (CC) of a conic surface is calculating the radius and CC with radii difference and Seidel coefficients obtained by comparing the test surface with a spherical reference wavefront having a ROC equal to the local sagittal,medial,or tangential ROC. The method is explained in detail and the angle of the optical axis and normal of the conic segment is introduced and divided into two angles α and β to improve the mathematical model. Angle β can be removed because of the symmetry of the segment according to the specific Zernike coefficients to simplify the new model. Equations about the vertex ROC and CC are derived with α and the simulation program is compiled. The maximum Seidel coefficients diffidence between theoretical calculation and simulations of a paraboloidal (=100 mm,F=3) is 0.0002 λ when α is 0.03° and β is 0. The research indicates that there are the same characteristic relations of the Seidel coefficients at the three positions even with the angle α.