The analytical expression for the propagation of an optical vortex dipole (OVD) nested in a Gaussian beam diffracted by a half screen is derived and used to study the evolution of the diffracted OVD. It is shown that Compared with the case of the OVD evolution in free space, multi-pairs of the OVD may take place in the diffracted field, and the annihilation fashion depends on the off-axis displacement. A variety of the evolution behaviors of OVDs appear by varying the off-axis parameter, but the topological charge is conserved in the evolution process.