The reaction mechanism and rate constant of the H2O2+Cl reaction, with and without a single water molecule, was investigated theoretically at the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ level of theory. The calculated results show that there is only one channel for the formation of HO2 + HCl in the naked H2O2+Cl reaction with an apparent activation energy of 10.21 kJ•mol-1. When one water molecule is added, the product of the reaction does not change, but the potential energy surface of the reaction becomes complex, yielding three different channels RW1, RW2, and RW3. The single water molecule in the RW1 and RW2 reaction channels has a negative influence on reducing the reaction barrier for the formation of HO2 + HCl, whereas it has a positive influence in Channel RW3. Additionally, to estimate the importance of these processes in the atmosphere, their rate constants were evaluated using conventional