As an alternative to the depletion of fossil resources, the reduction of CO2 emitted from fossil fuel combustion into valuable chemicals and fuel has drawn increasing attention. Due to the highly thermodynamic stability of CO2, it is still very challenging to find a sustainable and cost-efficient route to selectively convert inert CO2 at a high conversion rate under mild conditions. The water could also be a significant factor governing the reaction mechanism. On the one hand, water is one product accompanied with CH3OH. H2O might trigger the autocatalysis to accelerate CO2 activation and methanol production. On the other hand, Cu is also a highly active catalyst for water-gas shift (WGS) reaction and its reverse reaction (RWGS). The water is also possible to launch or to suppress the side reaction, which could influence the selectivity.