The　adsorption　of　HCHO　molecules　on　perfect　and　S-deficient　FeS2(100)　surfaces　was　studied　with　a　periodic　slab　model　by　Perdue-Burke-Ernzerhof　approach　of　General　Gradient　Approximation　within　the　framework　of　the　density　functional　theory.　The　calculated　results　show　that　HCHO　reacted　with　both　surfaces　through　the　O　atom.　HCHO　adsorbed　aslant　on　the　Fe-top　site　on　the　perfect　surface,　while　there　were　two　stable　structures　for　HCHO　on　the　S-deficient　surface,　where　HCHO　bonded　with　one　and　two　fourfold-coordinate　Fe　cations,　respectively.　The　calculation　of　density　of　states,　Mulliken　population,　and　vibrational　frequencies　of　the　adsorption　systems　indicated　that　the　electrons　transferred　from　the　substrate　to　HCHO,　and　the　bond　of　C=O　was　elongated　and　weakened.