Abstract: The occurrence mechanism of arsenic (As) impurity in pyrite (FeS₂) and its effect on the crystal structure and properties of pyrite were calculated by density functional theory (DFT) plane-wave pseudopotential method. The results showed that arsenic bearing pyrite was formed by As substitution for S. The incorporation of As would reduce the band gap and slightly increaseed the lattice parameters of pyrite. In arsenic bearing pyrite, As atom was positively charged. In addition, the incorporation of As element would affect the charge distribution of surrounding atoms and the strength of bond covalency between atoms. S and Fe atoms obtained electrons, and the covalency of As—S bond was stronger than that of S—S bond, while the As—Fe bond was anti-bonding. Through the density of states (DOS) analysis, It was found that As 4p orbital interacted with S 3p orbital and Fe 3d orbital.