在用氦氖激光器作为光源的夫琅禾费单缝及多缝衍射实验中，衍射光强分布曲线中央零级明纹处出现与理论图样不符的下凹的异常现象.考虑到衍射片镀膜层部分有微弱透光，本文构建了衍射片镀膜层透光条件下的非理想单缝/多缝衍射模型，并通过基尔霍夫衍射公式及高斯光束电场公式，推导得到高斯光束照射下非理想单缝/多缝衍射的衍射光强分布公式，利用数值模拟成功复现了观察到的异常现象，证实了其主要来源是缝附近镀膜层的微弱透光.

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.