In single-atom catalysts (SACs), the single atoms are often exposed as protrusions above the substrate. The solvent molecules in the electrocatalytic environment can interact or even bind to these coordination-unsaturated single atoms and thus influence the reaction process, but this has not been studied in depth. In this work, we systematically investigate the thermodynamics of CO₂ reduction reaction (CO₂RR) to CO over MoS₂-supported single metal atom catalysts (TM@MoS₂, TM = transition metal) under vacuum and explicit solvent environments using density functional theory. In addition, the ab initio molecular dynamics results show that explicit H₂O molecules can coordinate to the TM site and undergo competitive adsorption with the CO₂RR intermediates, which significantly affects the energy and conformation of the CO₂RR pathway. Electronic structure analysis reveals that the occupying H₂O molecules change the electronic state of single atom and further influence the adsorption strength of different CO₂RR intermediates. Our work shows that water molecules can not only act as ligands to influence the electronic state of TM, but also affect the energy and conformation of CO₂RR intermediates, which highlights the important role of occupying H₂O molecules at the single-atom sites in CO₂RR and provides useful insights for the design of SACs for efficient CO₂RR.