The uniformity of the pore size and the morphology of nano-porous anodic alumina (nano-PAA) can be improved by a pore-widening process. In this paper, the evolution of the pore diameter and the morphology are investigated in details and in real time. Phosphoric acid solution was used to chemically enlarge the pores of a nano-PAA prepared with oxalic acid. The pore-widening is monitored by carrying out simultaneously both top-view and cross-section analyses. From the top-view analysis, the pore diameter results to be gradually enlarged with the increasing etching time. However, such enlarging rate varies with the multilayer structure of the pore wall. From the cross-section analysis, the pores wall of a multilayered amorphous compound result to be etched in a stage-by-stage mode until almost everything is etched away. The pore-widening procedure on the upper portion is completed earlier respect to the lower portion, after some time the pore-widening becomes uniform along the axial direction. If the etching is maintained, hexagonal pore walls are destroyed and split into ultrahigh aspect ratio nanowires due to the etching anisotropy, and finally completely dissolved. This is the first experimental report on the structural evolution details of alumina nano-pores upon etching, combining analyses in the depth direction with stereoscopic 3D imaging. Results clarify the pore-widening mechanism in 3D nano-PAA and can have important practical applications.