The mechanism of vortex beam generated by spiral phase plate is expounded theoretically. The interference intensity expressions of the vortex beams, plane wave and spherical wave are derived and used to study the interference phenomenon of vortex beam. The corresponding relationship between topological charge and interference pattern is obtained numerically. Meanwhile, the vortex beams are obtained by spiral phase plate experimentally, and the corresponding experimental results are presented, as well as the experimental results and numerical results are practically consistent. The results show that vortex beams with different topological charges correspond to uniqueness interference pattern, hence the topological charge of vortex beam can be intuitively detected by the interference pattern.