夫朗和费衍射常用来测量微小尺寸，实验表明，衍射角较大时，存在较大的系统误差，测量结果不能正确反映被测量大小。本文以狭缝为例，采取一种新的近似处理方法，导出了较为准确的测量公式，分析了测量的正确度问题，指出夫朗和费近似只是必要条件，而要获得正确的测量结果，最大衍射角必须在一定范围内。同时，这种方法也不能测量任意微小尺寸，最小可测尺寸与所要求的正确度有关，要求的正确度越高，最小可测尺寸越大。上述结论同样适用于刻划光栅、金属丝网、徐层厚度及其它狭缝类或圆孔类物体的衍射测量。 Fraunhofer diffraction and diffraction commonly used to measure the small size, the experiment shows that the larger diffraction angle, there is a large systematic error, the measurement results can not correctly reflect the measured size. Taking slit as an example, this paper takes a new approximation method, derives a more accurate measurement formula, analyzes the accuracy of the measurement, points out that the Fraunhofer-sum approximation is only a necessary condition, and to obtain the correct measurement result, The maximum diffraction angle must be within a certain range. At the same time, this method can not measure any small size. The minimum size that can be measured is related to the required accuracy. The higher the required accuracy, the smaller the minimum size to measure. The above conclusion also applies to the diffraction grating, wire mesh, Xu layer thickness and other diffraction measurements of the slit or round hole objects.