Pyrite is the most common sulfide in nature, and it is well-known for its roles in acid mine drainage, flotation separation of useful metal (Cu, Pb, Zn, and Mo) sulfide minerals, optoelectronic and photovoltaic appliCation, pneumoconiosis, and even in the origin of life. However, the detailed oxidation behaviors of pyrite are still unclear and not well-understood. New oxidation pathways by O-2 on the pyrite (100) surface have been found in this work for the first time using density functional theory simulation; that is, besides Fe sites, S sites are also possible oxidation sites in the initial oxidation state of pyrite, where easier and stronger oxidation may occur. This is the first time to confirm the other researchers' conjecture on the direct oxidation of S sites, which explains the isotopic composition experiments that a minor amount of O-2 is permanently incorporated into SO42- during pyrite oxidation (O in SO42- is mainly derived from water). We constructed various H2O-O-2 coadsorption models on the pyrite surface by considering the adsorption sequence of H2O and O-2. It is found that the H2O molecule undergoes step-wise dissociation in the presence of the O-2 molecule. Hydroxyl radical center dot OH is the reactive oxygen species during H2O dissociation. Cyclic voltammetric measurements confirm the presence of center dot OH. In addition, 11202 may also be formed on the surface in terms of H2O-then-O-2 sequence adsorption.