The reactants, products, the transition states, and the intrinsic reaction coordinates are calculated to demonstrate the water splitting reaction process by employing the density functional theory. The results show that Pt-4 cluster can abstract H-2 from H2O molecule with different intrinsic reaction coordinates. The product O@Pt-4 can react with CO to produce CO2. Molecular dynamics simulation is used to demonstrate the escaping process of the CO2. The results show that the CO2 can completely escape from the Pt-4 cluster at 373 K and the alone Pt-4 cluster can be used to split H2O molecule again.