Coal is often adhered to by pyrite during slime flotation, causing an increase in the sulfur content of clean coal. In order to study the mechanism of pyrite adhesion to coal surfaces, different coal structural units were built and optimized, and the most stable adsorption model of them on pyrite surfaces was determined. The mechanism of pyrite particles adhering to the surface of coal slurries was explored with the method of DFT. The results showed that the interaction mechanism between pyrite surface and Ph-OH and Ph-O-CH₃ was the result of a weak interaction between the H atom of Ph-OH and Ph-O-CH₃ and the S atom of the pyrite surface. The interaction mechanism between the pyrite surface and Ph-COOH and Ph-CO-CH₃ was both as a result of H-S interactions and weak Fe-O interactions. On the whole, there were weak interactions between pyrite particles and the coal slurry, and the pyrite particles can spontaneously adsorb on the surface of the coal slurry.