Abstract: Flotation is based on the difference of wettability of mineral surfaces. In order to study the water wettability mechanism of pyrite and coal surfaces, pyrite and perfect coal surface models were constructed. The adsorptions of water and oxygen molecules on pyrite and perfect coal surface were analyzed by density functional theory (DFT). The results show that the electrons of pyrite surface are more active. There are hanging bonds on the Fe and S atoms which make the pyrite surface have strong adsorption activity. The adsorption energy of water molecules on the pyrite surface is negative value. The adsorption configuration of water molecules on the sulfur acupoint (adsorption energy is −87.42 kJ/mol) is the most stable. Oxygen molecules adsorbed on the pyrite surface can dissociate, but it has little effect on the adsorbed water molecules. The coordination number of atoms on the perfect coal surface is the same as the bulk phase. The binding of electrons on the surface is strong, which makes the adsorption activity on the perfect coal surface weak. The adsorption energy of water molecules on each adsorption site is positive value, which indicates that water molecules are difficult to adsorb on the surface. Oxygen molecules cannot dissociate when adsorbing on the perfect coal surface, and the adsorbed water molecules are discharged from the surface. Therefore, pyrite surface has strong hydrophilicity, while perfect coal surface has strong hydrophobicity.