Alignment error of the pinhole is the main error factor that affects the quality of diffraction reference wavefront in a point diffraction interferometer. Based on Rayleigh-Sommerfeld vector diffraction theory, a theoretical model of non-paraxial Gaussian beams diffracted through a pinhole is established, and diffraction wavefront error caused by different alignment errors is numerically analyzed. The influence of focusing spot size is especially considered. The research results show that the introduction of even a small alignment error can make the diffraction wavefront error increases quickly, diffraction wavefront error increases linearly as the alignment error increases; under same alignment error, the bigger the focusing spot size, the smaller the diffraction wavefront error. In order to reach the same level of diffraction wavefront accuracy, the allowable alignment error increases nearly in same multiples as the focusing spot size increases. Diffraction wavefront error distribution of 0.5~3 μm pinholes in different spot size and alignment error are acquired in the analysis, it can provide favorable reference data for the selection of focusing lens, determination of pinhole alignment accuracy requirements and the evaluation of diffraction wavefront error in certain condition.