The chemical mechanism of the H2S + NO3 reaction in the absence and presence of water molecules was investigated using the Born-Oppenheimer molecular dynamics. These calculations were performed to gain insight into the underlying chemical mechanism and to evaluate the kinetic isotope effect in the H2S + NO3 and D2S + NO3 reactions. When H2O interacts with NO3, the rate coefficient of the H2S + NO3 reaction is smaller than that for H2O interacting with H2S. Deuterium generally decreases the rate when D2O interacts with D2S but has no effect when D2O interacting with NO3. When H2O or D2O interacts with NO3, the yields are larger compared to those for the reactions (H2O)H2S + NO3 and (D2O)H2S + NO3. Furthermore, the average reaction times of the reactions H2S + NO3(H2O) and H'S + NO3(D2O) are shorter than those when H2O or D2O interacts with H2S. The (H2O)H2S + NO3 reaction may occur via two possible pathways: the non-water-assisted and water-assisted hydrogen abstraction mechanisms. However, the H2S + NO3(H2O) reaction only happens via the non-water-assisted mechanism.