The Kirchhoff diffraction theory is applied to the four-pinhole aperture diffraction screen, so that the intensity, the zero contour of the real and imaginary parts of complex amplitude and the phase distribution in deep Fresnel diffraction region are simulated, and it is found that the bright spots in diffraction field show central symmetric distribution. When the observation plane closes to the diffraction screen, the zero-value points of light intensity can form line segment, on which the eccentricities of the light intensity isoline are close or equal to 1, and the intensity changes very fast on both sides of the zero line of light intensity. The zero contours of the real and imaginary parts of complex amplitude are closed curves. The number of intersection points of the zero contour is even, and positive and negative singularities are equal. Not only the phase around special phase singularities appears symmetric distribution, but also the topological charges of special phase singularities equals zero. With the propagation of the optical wave, the line segment of zero-value intensity changes to be shorter and shorter, finally to be a point.