Abstract: The reaction behavior of pyrite in process of Bayer digestion were studied in this paper.The thermodynamics analysis showed that the gibbs free energy of reaction between pyrite and alkali solution was negative and decreased with the raising of temperature in range from 373 K to 573 K, and the sulfur as the form of pyrite can easily react with alkali liquor.The experiment researches indicated that there were more than half of the sulfur dissolved into solution in pre-desilication process, and the dissolution rate of sulfur was up to 90 % in the high-pressure digestion process.When the temperature dropped, the sulfur in solution was precipitated as a kind of dark green material, which was hydroxyl-Fe polymers(Na₂FeS₂(OH)₂·2H₂O).