The phase and the zero-contour of the real part and the imaginary part of the interference field on a far-field plane generated by multi-aperture diffraction screen are simulated. It is found that when the orbital angular momentum quantum number of incident beam is equal to zero, at the center of interference field the zero-lines cannot cross each other, therefore, thereby the phase vortices cannot form. When the orbital angular momentum quantum numbers of incident beam are opposite to each other in sign, namely -1 and +1 at the center of interference field the zero-lines are perpendicular to and cross each other, the signs of the phase vortices at the corresponding positions in interference fields are also opposite to each other. When the orbital angular momentum quantum numbers of incident beam are equal to ±2 and ±3, there are four zero-lines that cross each other at the center of interference fields, where the topological charge values of phase vortices are just equal to the orbital angular momentum quantum numbers of the Laguerre-Gaussian beam. Therefore, these results can be used to measure the orbital angular momentum of optical vortex beam.