The Cu-Sn binary catalysts are found to selectively reduce CO2 to CO electrochemically with promising efficiencies. The issue of active sites is not resolved yet. In this paper, first-principles calculations are performed to study the stability, as well as the hydrogen evolution and the CO2 reduction properties, of the Sn-modified Cu surfaces. It is revealed that a special type of Cu step edges with inner atoms substituted by Sn single atoms can not only keep the CO2 reduction property of a pure Cu edge but also inhibit the H2 formation, while being stable enough under reduction conditions. By analyzing the electronic properties of the edges, we found that the Sn atom among many others can make the most optimized effect in enhancing the catalytic selectivity of the Cu surface.