A detailed theoretical study was done in order to clarify the reaction mechanisms of the singlet dibromocarbene (1CBr2) with 3O2 on the singlet potential energy surface (PES). All the geometries of reactants, intermediates, transition states and products were obtained at the B3LYP/6-311++G(d,p) level. Intrinsic reaction coordinate (IRC) calculations at the same level were carried out to confirm the connections between transition states and intermediates. It is found that the initial adduct Br2COO (Cs) is formed via a barrierless association in the 1CBr2 + 3O2 reaction, and then some isomerizations and breakages of bonds take place, generating P1 (BrCO + BrO), P2 (CO + Br2O), P3 (CO2 + Br2) and P4 (CO2 + 2Br). P3 (CO2 + Br2) is the most competitive channel kinetically and thermodynamically. P4 (CO2 + 2Br) is the least favorable one kinetically.