A second-order Moller-Plesset perturbation theory (MP2) and density functional theory (DFT) investigation of the dehalogenation reactions of thionyl chloride is reported, in which water molecules (up to seven) were explicitly involved in the reaction complex. The dehalogenation processes of thionyl Chloride were found to be dramatically catalyzed by water molecules, The reaction rate became significantly faster as more water molecules became involved in the reaction complex. The dehalogenation processes can be reasonably simulated by the gas-phase Water cluster Models, which reveals that water molecules can help to solvate the thionyl chloride molecules and activate the release of the Cl- leaving group. The computed activation energies Were Used to compare the calculations to available experimental data.