Abstract: In the traditional methods of computing the OTF,it is general that the transversal magnification is supposed to be not changeable in the whole aperture,though the anamorphic imaging produces considerable errores in the calculation of the OTF.People do not still find a good method to solve the problem.Starting from the Kirchhoff diffraction integral,the classical formula of the OTF in the Fraunhofer approximation is a new derived in the exit pupil coordinates and in the entrance pupil coordinates.Using piecewise polynomial interpolation,the OTF in the exit coordinates is studied for anamorphic imaging.It also gives comparison with the OTF formula of in the Fresnel approximation.