Objective To analyze the surface defects of the aluminum alloy substrate after chromic acid anodizing and its cause.Methods The morphology, microstructure and corrosion products of the defects were analyzed by scanning electron microscopy ( SEM) , metallurgical microscopy and energy dispersive spectrometer ( EDS) , respectively. Results The defects on the substrate of the aluminum alloy could be caused by the intergranular corrosion when treated by chromic anodizing. Conclusion The elements of Cu, Ni, Mg and Fe could be enriched to form segregation in the process of heat treatment to cause the corrosion of substrate in the chromic acid solution.