Abstract: The two-photon processes based optical storage is one of the most promising three-dimensional technologies for increasing the capacity of data storage. In order to compute the diffraction patterns within the storage medium produced by a single-beam-two-photon recording system, we have set up a mathematical model by using the techniques of ray-tracing and amplitude-combination. This model gets rid of the limitation in Fresnel or Fraunhofer approximations. So it is valid for the objective lens with a large numerical aperture. The numerical simulation results in this paper show that our model can rapidly compute the intensity distribution in different recoding planes in a storage medium.