Solid sorbents based CO2 capture has become increasingly important. Great progress has been achieved with experimental studies in this area. However, the density functional theory based capture study on the function of H2O in CO2 capture is lacking. This research was designed to make progress in this important area with TiO2-supported K2CO3 being an example. Due to its high cost-effectiveness, dry K2CO3 is a promising sorbent for capturing CO2. Yet challenges remain in accelerating the rate of the absorption process. The study of mechanism of the effect of H2O on CO2 adsorption as well as the carbonation reaction can help select and design better support for the sorbent. Up to now, it is open. In this work, the adsorption and reaction of CO2 over K2CO3 loaded on a rutile (1 1 0) surface have been studied using theoretical calculations. The results show that the CO2 adsorption is increased when H2O appears, and carbonation reaction mainly occurs at the interfaces of K2CO3/TiO2 includes bicarbonate formation resulting from the reactions of CO2 with OH via H2O dissociation and CO3 anion with transferred H via H2O dissociation combining. In addition, H transfer step appears when support TiO2 exists compared to that on pure K2CO3 sorbent. The kinetic modeling indicates that the H2O dissociation may limit the carbonation reaction. Therefore, H2O-dissociative or high OH coverage TiO2 support material can assist CO2 sorption with the solid K2CO3 based CO2 capture technology. It is expected that the theoretical study sheds light on the preparation of cost-effective CO2 sorbents in the future.