Abstract: Employing density functional method (B3LYP) with the relativistic effective core potential (RECP) for Pu atom and all-electron basis set aug-cc-pVTZ for O and H atoms, the equilibrium geometrical structures of PuO₂ molecule were optimized. In addition, four structures for Pu-water were proposed and studied. The results indicate that water molecule tends to dissociate to form more stable structure with Pu. Due to the lager difference of electronegativity for Pu and O atoms, the stable structure is from the interaction of Pu and O atoms. The thermodynamic functions of adsorption and dissociation reactions of Pu and H₂O were calculated according to electronic-vibration approximation. The results show that the water vapor molecule can’t adsorb on Pu surface even at low temperature, in contrast, dissociation reactions of H₂O on Pu surface can occur spontaneously.