Abstract: The adsorption mechanism of water molecule on the molecular surface of different rank coals was investiga- ted by the density functional theory with dispersion correction ( DFT-D3) in the quantum chemistry method. Anthra- cite,bituminous,subbituminous and lignite were selected as the typical coal ranks. The equilibrium configurations of water molecule adsorbed on the surface of coal molecules were optimized,the analysis of electrostatic potential on the surface of coal molecule was conducted,and the interaction energy between coal molecular surface and water was cal- culated,also,the interaction types of water molecule on the molecular surface of different rank coals were confirmed by calculating the reduced density gradient function (RDG). Research indicates that the maximum interaction energy be- tween coal molecules and water molecule can be -11. 91 kcal / mol which confirms that the adsorption process of water molecule on coal molecule surface belongs to physical adsorption,and the interaction energy depends on the type and number of weak interaction forces. Lignite has the largest molecular surface area with large electrostatic potential,re- vealing that lignite is more likely to interact with polar water molecule. The maximum interaction energy of four differ- ent rank coals molecule adsorbed water molecule can be in the order of lignite > subbituminous > bituminous > anthra- cite. The interaction between anthracite and water molecule belongs to van der Waals force;the interaction between bituminous and water molecule is hydrogen-bonding effect;the interaction between subbituminous and water molecule is a joint contribution of van der Waals force and hydrogen-bonding;and the interaction between lignite and water is the combined effect of two strong hydrogen bonds.