There remain considerable challenges for Cs-based catalysts in gas-phase dehydrochlorination of 1,1,2-trichloroethane (TCE) due to their poor catalytic performance and harsh reaction temperature requirements. In this work, we report a novel supported ionic liquid (IL) phase Cs-based system and evaluate its catalytic performance and mechanism, combining the kinetic and DFT calculations. The optimal Cs-IL/SiO₂ catalyst achieved the TCE conversion of 60% and the selectivity to 1,1-Dichloroethylene (VDC) of 86% under the mild temperature of 200°C. It is demonstrated that IL additives can significantly improve the dispersion of Cs species during the catalyst preparation and evaluation process. Moreover, IL promotes the activation of TCE and enhances the desorption efficiency of HCl, which is the main reason for its high activity. The selectivity is improved by changing the adsorption behaviors of the products, which ensures the preferential formation of the main product VDC. The enhanced activity and selectivity of Cs-IL/SiO₂ suggest a promising candidate for dehydrochlorination reaction.