现代精密光学系统的发展，对光学元件的加工和检测提出了非常严格的要求，达到纳米量级。相移式点衍射干涉仪是一种应用在纳米精度检测中的常用干涉仪，其参考波前由直径在几百纳米量级的小孔衍射产生，其衍射波前与理想波面的误差，决定了干涉仪的检测精度。基于有限元方法，计算了聚焦入射情况下，不同直径小孔的衍射波面。分析了聚焦斑发生对准误差和倾斜误差下，对衍射波面的影响。

The modern optical systems have placed stringent requirements on the manufactures and measurements of optical elements. Measurement accuracy of nanometer is needed. Phase-shift point diffraction interferometer is a common instrument in high-accuracy measurement, whose reference wave is generated by a pinhole with diameter of several hundred nanometers. So the measurement accuracy can be estimated by analyzing the diffracted reference wave. A two-dimensional simulation, based on finite element method (FEM), is set up to study the propagation of the visible light of 632.8 nm wavelength, through sub-1000 nm diameter pinholes in a chromium membrane with different thicknesses. Beam spot alignment error and tilt are also analyzed.