Arsenic　(As)　frequently　exists　in　pyrite　(FeS2)　in　the　form　of　impurities.　The　oxidation　behavior　of　As　in　FeS2　is　important　in　environmental　science,　mineral　processing,　and　other　related　fields.　The　adsorption　behaviors　of　H2O　and　O2　molecules　on　the　As-bearing　pyrite　(100)　surface　(As-FeS2(100))　are　studied　using　the　density　functional　theory　(DFT).　The　results　show　that　As　prefers　the　S　site　on　the　pyrite　(100)　surface　(FeS2(100)).　In　the　absence　of　O2,　an　isolated　H2O　molecule　does　not　dissociate　when　adsorbed　at　an　iron　(Fe)　site　and　is　repelled　at　an　As　site.　Furthermore,　the　surface　area　around　the　As　atoms　exhibits　a　hydrophobic　behavior.　Adsorption　energy　analysis　reveals　that　the　presence　of　As　atoms　is　unfavorable　for　the　adsorption　of　H2O　molecules　on　the　pure　FeS2　surface,　and　that　the　adsorption　of　H2O　molecules　on　the　As-FeS2(100)　is　physical　adsorption.　In　the　absence　of　H2O,　it　is　suggested　that　the　O2　molecule　easily　dissociates　on　both　the　pure　FeS2(100)　and　As-FeS2(100).　The　adsorption　of　O2　on　the　As-bearing　surface　is　weaker　than　that　on　the　pure　FeS2(100).　For　the　co-adsorption　of　H2O　and　O2,　the　adsorption　energy　on　the　As-bearing　surface　is　more　negative　than　that　on　the　pure　surface.　This　indicates　that　the　presence　of　As　promotes　surface　oxidation.　Additionally,　two　-OH　and　O　(AsO　or　SO)　or　-O　(Fe-O)　species　are　formed　on　the　surface　of　pyrite　when　the　H2O　molecule　is　dissociated.　The　ability　of　the　H2O　molecule　to　adsorb　on　the　pyrite　surface　is　decreased,　due　to　the　As　atom.　Additionally,　the　presence　of　the　Arsenic　(As)　impurity　weakens　the　adsorption　of　the　O2　molecule　on　the　pyrite　surface.　However,　the　results　indicate　that　the　presence　of　As　will　promote　surface　oxidation　during　the　co-adsorption　of　both　water　and　oxygen　(H2O-O2).image