Based on electron and vibration approximate means and the density functional theory B3LYP, with LanL2DZ basis sets for Pt and 6-311G** basis sets for hydrogen and oxygen, the different structures of Pt-OH₂ have been optimized. Pt and H₂O of stable Pt-OH₂ molecule are not in the same plane, and Pt atom is in connection with O atom directly. The thermodynamic functions and the equilibrium pressures of adsorption reactions of water vapor have been calculated, and their relationships with temperature were obtained. Above 298.15 K, ΔG⁰＞0 kJ•mol^－1 in 100～898.15 K, and the adsorption of water vapor molecule was not stable on Pt surface. Under 200 K, ΔG⁰＜0 kJ•mol^－1, and the water vapor molecule could adsorb on Pt surface steadily. ΔG⁰ and the equilibrium pressures of dissociation reactions have also been calculated. Above 298.15 K, ΔG⁰＞0 kJ•mol^－1. In 100～898.15 K, the water vapor molecules have no tendency to dissociate on Pt surface, and they participate in most reactions in intact molecule.

Key words: thermodynamic function, density functional theory, Pt, water vapor, adsorption, dissociation