The reaction mechanism and rate constant for the major channel between HO2 and H2S,without and with a single water molecule,has been investigated theoretically at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(2df,2p)level of theory.The calculated results show that the HO2 + H2S reaction mainly occurs through the channel of H2O2+HS formation with the apparent activation energy of 14.94 kcal·mol-1.When one water molecule is added,the reaction mechanism of the major channel becomes quite complex yielding three different reaction channels of H2O…HO2+H2S(RW1),HO2…H2O+H2S(RW2)and.H2O…H2S+HO2(RW2).For single water-catalyzed the major channel of HO2+H2S reaction,Channel RW1 is dominant and Path RW1 is the most favorable.Additionally,to estimate the importance of Channel RW1 in the atmosphere,its rate constants are evaluated using the conventional transition state theory with Wigner tunneling correction.The calculated results show that the effective rate constant of Path RW1 increases with the temperature increases.At 298 K,the value of kRW1/ktotal is up to 47.1%,indicating that the single water molecule has a obvious effect on the major channel of atmospheric reaction of HO2+H2S.The present study provides further insight into water effects in the atmospheric chemistry.