The surface layer of the bronze mirror "Hei Qi Gu" is composed of non-metal layer and transition layer. The valent electron structure analysis points out that the main phase of tran-sition Sn1-xCuxO2 is similar to SnO2 in stablility. It was found from study of experimental data that there is the coincidence site lattice structure at the phase boundary between the δ-phase Cu-Sn alloy and Sn1-xCuxO2 in the transition layer. This structure reduces the energy of Cu atoms at the phase boundary and makes the δ-phase Cu-Sn alloy stable, which is the main reason for corrosion resistant of the "Hei Qi Gu".