The formic acid catalyzed gas-phase reaction between H2O and SO3 and its reverse reaction are respectively investigated by means of quantum chemical calculations at the CCSD(T)//B3LYP/cc-pv(T+d)z and CCSD(T)//MP2/aug-cc-pv(T+d)z levels of theory. Remarkably, the activation energy relative to the reactants for the reaction of H2O with SO3 is lowered through formic acid catalysis from 15.97 kcal mol−1 to −15.12 and −14.83 kcal mol−1 for the formed H2O⋅⋅⋅SO3 complex plus HCOOH and the formed H2O⋅⋅⋅HCOOH complex plus SO3, respectively, at the CCSD(T)//MP2/aug-cc-pv(T+d)z level. For the reverse reaction, the energy barrier for decomposition of sulfuric acid is reduced to −3.07 kcal mol−1 from 35.82 kcal mol−1 with the aid of formic acid. The results show that formic acid plays a strong catalytic role in facilitating the formation and decomposition of sulfuric acid. The rate constant of the SO3+H2O reaction with formic acid is 105 times greater than that of the corresponding reaction with water dimer. The calculated rate constant for the HCOOH+H2SO4 reaction is about 10−13 cm3 molecule−1 s−1 in the temperature range 200–280 K. The results of the present investigation show that formic acid plays a crucial role in the cycle between SO3 and H2SO4 in atmospheric chemistry.