A systematic DFT study on production selectivity of Cu single-crystal surfaces are performed at electrochemical interfaces to gain the origin of CO2 electroreduction into C-1 and C-2 species. The results show that CO is a common intermediate on Cu(1 1 1) and Cu(1 0 0) surfaces, and its further reduction is the crucial selectivity-determining step. Simultaneously, a Langmuir-Hinshelwood mechanism is proposed for CO dimerization and the reason for pathway selectivity is explained by geometry analysis. The present studies will provide theoretical support for C-1 and C-2 species formation mechanisms and the rational design of Cu-based alloy electrocatalysts that are active at a lower overpotential.