In this paper,we introduce a new type of double cross aperture.The Fraunhofer diffraction patterns of a vortex beam through different double cross apertures are analyzed theoretically and experimentally.It is found that the pattern of the far-field diffraction intensity distribution displays a rectangular lattice array,which becomes much clearer by choosing suitable width,length and center of the double cross slit.The number of the light spot points of all external sides of the rectangular lattice array divided by two is just equal to the topological charge value of the measured optical vortex.Based on this property,a simple and feasible method is provided to detect the topological charges of an optical vortex beam.