The reaction mechanism of CH3O2 and HS was systematically investigated by density functional theory (DFT). Six singlet pathways and seven triplet ones are located on the potential surface (PES). The result indicates that the main products are CH3O and HSO both on the singlet and triplet PES, different from the CH3O2 + OH reaction. Moreover, deformation density (ρdef) and atoms in molecules (AIM) analyses were carried out to further uncover the nature of chemical bonding evolution in the primary pathways. Furthermore, reaction rate constants were calculated in the temperature range from 200 to 1000 K using the transition state theory with the Wigner and Eckart tunneling corrections. Our results can shed light on the title reaction and offer instructions for analogous atmospheric reactions, as well as experimental research in the future.