In the experiment of observing Fraunhofer single-slit and multi-slit diffraction with He-Ne laser as light source，an abnormal concave shape appears at the center of the diffraction intensity distribution curve，which is inconsistent with the theoretical pattern. Considering the weak light transmission of the coating layer of diffraction plate，a non-ideal single-slit/multi-slit diffraction model is constructed under the condition of extra light transmission in the coating layer. Based on Kirchhoff diffraction formula and Gaussian beam formula，the formulas of single-slit and multi-slit diffraction of Gaussian beam under the above model are derived，and the observed abnormal phenomenon is successfully reproduced through numerical simulation. It indicates that the main cause of this phenomena is the weak light transmission of coating layer near the slit.