Starting from the Kirohhoff diffraction integration, a new formula is derived with the Fresnel approximation for computing the OTF by means of exit pupil coordinates. In numerical method, a series of measures are taken to improve the accuracy of computation and reduce the volume of computation. It included that the wave aberration function is calculated by interpolation with its derivatives, the optical pupil shape is described by section interpolation of quadratic polynomial, the Simpson formula is used for making numerical integration, a differential expression for the position of the focal plane to move and. an algorithm for approximating the position of the optimum focal plane are given too. Finally, there are two numerical examples to show that the now method is available.