In view of the lack of the detailed analysis and elaboration on the Fresnel diffraction pattern of single slit and its transition to Fraunhofer diffraction pattern in the existing textooks or reference books，a proposal is given on designing a diffraction simulation experimental section between learning the diffraction theory and conducting an experimental verification. For this purpose，the mathematic formula used to realize the simulation is deduced from the diffraction theory. Then，taking the single slit diffraction as an example，Matlab programming and its graphical user interface ( GUI) module are used，and a diffraction simulation experiment is designed. Through analyzing the results of the simulation experiments，the charateristics of the two diffraction phenomena and their transition situations are elaborated in detail. Finally，the real experiment is carried out by using actual experimental devices to verify the reliability of the simulation experiment and the advantages of introducing such a section. It is proved that the simuluation experimental section provides a useful auxiliary means for better guiding students to study the diffraction of light in depth.