In this article, the reaction mechanisms of H2S + (3)O2 formation by the HO2 + HS reaction without and with catalyst X (X = H2O, (H2O)2 and (H2O)3) have been investigated theoretically at the CCSD(T)/6-311++G(3df,2pd)//B3LYP/6-311+G(2df,2p) level of theory, coupled with rate constant calculations by using conventional transition state theory. Our results show that in the presence of catalyst X (X = H2O, (H2O)2 and (H2O)3) into the channel of H2S + (3)O2 formation, the reactions between the SH radical and HO2(H2O)n (n = 1-3) complexes are more favorable than the corresponding reactions of the HO2 radical with HS(H2O)n (n = 1-3) complexes due to the lower barrier of the former reactions and the higher concentrations of HO2(H2O)n (n = 1-3) complexes. Meanwhile, the catalytic effect of water, water dimers and water trimers is mainly taken from the contribution of a single water vapor molecule, since the total effective rate constant of HO2H2O + HS and H2OHO2 + HS reactions was, respectively, larger by 7-9 and 9-12 orders of magnitude than that of SH + HO2(H2O)2 and SH + HO2(H2O)3 reactions. Besides, the enhancement factor of water vapor is only 0.37% at 240 K, while at high temperatures, such as 425 K, the positive water vapor effect is enhanced up to 38.00%, indicating that at high temperatures the positive water effect is obvious under atmospheric conditions. Overall, these results show how water and water clusters catalyze the gas phase reactions under atmospheric conditions.