Solid liquid interface, which is the location that electrochemical reaction occurs, is essential for CO2 reduction. However, different theoretical calculations often use different models of solid liquid interface and predict conflicting mechanisms, e.g. for the formation of HCOOH. To address this issue, we adopt different structures of solid liquid interface to mimic the reaction circumstance using density functional theory calculations. We find that the reaction barriers and energies are sensitive to the reaction circumstance, such as the state of water molecules, the number of water molecules, and the available surface sites, whereas the selectivity of CO2 reduction can be hardly changed once a water bilayer network on Cu surface is used. In particular, a solid liquid interface built from 4-6-4 water rings (5H(2)O/1H) can present an appropriate description of the solvent, predicting the formation pathways of HCOOH in reasonable agreement with the experiments. These results confirm the role of water bilayer networks in describing solid liquid interface in the electrocatalysis, which serves as an important basis for the future studies.