The absorptive characters of O2 on Fe31MnC(001) surface at three different sits (top, bridge and hollow) and effect of C on the magnetic properties of Fe31Mn has been investigated by using the generalized gradient approximation (GGA) based on density functional theory (DFT). The results indicate that O2 tend to be vertically over the four-coordination vacancy hollow site. For Oxygen atom adsorption, it has a larger adsorption energy when oxygen atoms occupy the adjacent four-coordination vacancy hollow site. Among several possible adsorption configurations considered, oxygen at bridge site on Fe31MnC(001) surface is 3.8 eV, which is the most favorable adsorption site. Meanwhile, Carbon atom not only increase the magnetization hybridization of matrix, but also inhibit antiferromagnetic interaction of the adjacent Fe and Mn atoms.