The refinement mechanism of alternating current pulse (ACP) on the solidification macrostructures of pure Al and the characterization of refining efficiency were investigated by embedding the wire mesh in the mold. The experiment results showed that ACP treatment during solidification led to the formation of fine equiaxed grain. There were remarkably differences with respect to the area of fine equiaxed zone inside and outside the tube. Lorentz force, induced melt flow and the rest of intrinsic effects of ACP inside and outside the tube were discussed in the present study. It demonstrated that the forced melt flow could lead to the columnar fragmentation and make the crystal nucleus on the mold wall fall off and drift in the liquid, leading to grain refinement. In addition, Reynolds number was suitable to characterize the refining efficiency of pure Al under ACP.