In order to determine the topological charge of a Perfect Optical Vortex (POV), a in-line interferometric measurement method was presented. The basic idea is to use a spatial light modulator to produce a POV and a spherical wave simultaneously. By modulating the divergence angle of the spherical wave, the POV and the spherical wave interfer. The number of interference fringes is used to realize the direct and rapid measurement of topological charge. Simulated and experimental results show that the interference fringes obtained by the method can be used to determine the topological charge of POVs, including the magnitude and symbol. Furthermore, the proposed in situ method is demonstrated to determine the topological charge of the POV array by measuring the interference pattern of a perfect vortex array and a spherical wave. The proposed in situ method is simple and effective. Therefore, it is of a significance for the application of orbital angular momentum control and information coding based on POVs.