In order to solve the problem of incomplete polishing of the irregular waveguide inner sphere by abrasive flow polishing, the copying mold cores were set to guide the abrasive flow through the inner sphere and polish it. Fluid simulations for channel models formed by different copying mold cores and waveguide inner cavities have been carried out by Fluent, through which the flow velocity, the pressure and the wall shear stress of the equivalent wall have been contrasted and analyzed. The simulation results revealed that the machining quality of inner sphere is better with the increase of the ball diameter of the mold core. The abrasive flow polishing test has been carried out with a 9mm ball diameter copying mold core, after that the surface roughness S_a of the waveguide inner sphere is reduced from 5.07μm to 0.51μm and the surface quality is improved significantly.