The topological charge variation of a vortex beam passing through a diffraction plate with a Fermat spiral microporous structure and the diffraction and focusing characteristics of the beam intensity distribution are studied theoretically and experimentally. A positive lens is used to focus the diffracted beam, and the variation of the intensity distribution behind the focal plane is observed and studied. The intensity distribution of the innermost ring of the diffracted beam has about five stages before and after the beam waist. This variation trend is applicable to the case of the diffracted plate with different Fermat spirals distributed by the vortex beam through the microhole, whether the rotation direction of the spiral is changed or the number of spiral structures is changed. The change of topological charge is verified by the interference between diffraction beam and spherical wave. The results show that the diffraction can generate a new topological charge, which is related to the phase front of the incident vortex beam and the relative rotation of the helical microhole array.