In the construction of the pinhole point diffraction interferometer, the alignment error between the convergent spot of the microscopic objective lens and the diffraction hole in the front end of the pinhole diffraction will lead to problems such as diffraction wavefront error, diffraction intensity reduction, and interference fringe contrast reduction, which will affect the actual measurement accuracy. In order to solve the problem of inaccurate alignment between the convergent spot of the microscopic objective lens and the diffraction hole, a diffraction hole visual alignment method based on the auxiliary optical path is proposed in this work. An auxiliary alignment optical path is built at the front end of the pinhole diffraction, and the beam reflected by the pinhole diffraction plate is mainly reflected by the beam splitter prism, and then received by a charge coupled device (CCD). By collecting and processing the spot image reflected by the small hole diffraction plate, the alignment state of the small hole is monitored and the alignment error is calculated. In this work, a visual-precision optical path alignment scheme is designed, and the visual performance of the alignment image under three typical alignment deviations of translation, tilt and defocus is simulated and analyzed. The mathematical model of the object-image relationship between the alignment image and the alignment error is constructed, and the alignment image error measurement and processing algorithm is studied. The experimental results show that the auxiliary optical path alignment method and the alignment image processing algorithm proposed in this work are feasible, and the alignment accuracy can reach 0.05 μm. The research results are helpful in improving the alignment efficiency and accuracy of point diffraction interferometer, and can lay a certain technical foundation for the development of practical point diffraction interferometer.