小孔的直径影响小孔衍射波前的误差，进而会影响点衍射干涉(PDI)检测的精度。基于矢量衍射理论中的时域有限差分(FDTD)方法，精确分析了可见光情况下衍射小孔直径接近以及小于入射光波长时所对应的衍射波前相对于标准球面波的偏离误差。数值仿真结果表明，当小孔的直径为400 nm时，所对应的数值孔径为0.65的衍射波前的误差峰谷值小于1 nm，而误差均方根值小于0.35 nm。随着小孔尺寸的减小，其衍射波前的误差进一步减小。仿真结果原理上验证了小孔点衍射干涉方法用于实现大数值孔径球面的高精度检测的可行性，并为实际检测中点衍射小孔尺寸的选择提供了精确的数值依据。

The dimension of pinhole affects the error of diffraction wavefront, which is the critical element that determines the precision of point diffraction interferometer (PDI). The deviation of diffracted wavefront from an ideal sphere at visible wavelength, which is aroused by the dimension of the pinhole, is numerically analyzed based on the finite difference time domain (FDTD) method. The simulation results show that the peaktovalley (PV) value of diffraction wavefront error is less than 1 nm and the root mean square (RMS) value is less than 0.35 nm when the diameter of pinhole is 400 nm and numerical aperture (NA) is 0.65. The high precision of testing spherical surfaces with high NA is demonstrated. The analysis of pointdiffraction wavefront error with FDTD method provides theoretical basis for choosing the dimension of pinhole in experiments and tests with PDI.