In a point diffraction interferometer (PDI), the shapes and positions of pinhole diffraction spots are determined by pinhole alignment conditions, and then the arrangement of subsequent optical path is affected. Based on Rayleigh-Sommerfeld diffraction theory, the pinhole diffraction intensity distribution for misaligned Gaussian beam incidence is studied. The mathematical expressions of different alignment errors including lateral shift, defocus, and tilt are given. The intensity distributions of pinholes with different diameters for different alignment errors are analyzed. The research results show that lateral shift alignment error does not affect the center position of diffraction spot, but it will change the shape of diffraction spot. With the increase of lateral shift alignment error, the first diffraction dark ring is with a crescent shape and gradually disappears in the direction of shift. Defocus alignment error has no obvious effect on position and shape of diffraction spots, but the intensity of diffraction light will decrease with the increase of defocusing amount. Tilt alignment error will cause the offset of the center position of diffraction spot, but it will not affect the shape of the diffraction spot. The offset direction is consistent with oblique incidence direction, and there is a linear relationship between the offset and tilt angle.