CuO based electrocatalysts generally exhibit better selectivity for C products (ethylene or ethanol) in electrochemical carbon dioxide reduction. The surface characteristic of the mixed Cu and Cu chemical state is believed to play an essential role that is still unclear. In the present study, density functional theory (DFT) calculations have been performed to understand the role of copper chemical states in selective ethanol formation using a partially reduced CuO surface model consisting of adjacent Cu/Cu sites. We mapped out the free energy diagram of the reaction pathway from CO intermediate to ethanol and discussed the relation between the formation of critical reduction intermediates and the configuration of Cu/Cu sites. The results showed that Cu sites facilitate the adsorption and stabilization of *CO, as well as its further hydrogenation to *CHO. More importantly, as compared to the high reaction energy (1.23 eV) of the dimerization of two *CO on Cu/Cu sites, the preferable formation of *CHO on the Cu site makes the C-C coupling reaction with *CO on the Cu site happen under a relatively lower energy barrier of 0.58 eV. Furthermore, the post C-C coupling steps leading to the formation of the key intermediate *OCHCH to C compound are all thermodynamically favoured. Noteworthily, it is found that *OCHCH inclines to the ethanol formation because the coordinatively unsaturated Cu site could maintain the C-O bond of *OCHCH, and the weak binding between *O and Cu/Cu sites helps inhibit the pathway toward ethylene. These findings may provide guidelines for the design of CO and CO reduction active sites with enhanced ethanol selectivity.