Abstract: Thermodynamics of synthesis of 3,6dichlorosalicylic acid by KolbeSchmitt reaction was studied by using the B3LYP method of density functional theory. All of the structures of the reactants and products were optimized under 6311++g（3df，3pd）level. The changes in reaction enthalpy, Gibbs free energy and equilibrium constant under reaction temperature (300—600 K) for the primary and side reaction were calculated and analyzed with the basic thermodynamic data obtained from vibration frequency analysis. The result shows that KolbeSchmitt reaction processes are exothermic but the primary reaction can’t spontaneously occur at low temperature and low pressure while the side reaction is entirely feasible on thermodynamic which can proceed easily under mild conditions. In addition the KolbeSchmitt reaction is reversible that the equilibrium conversion could be promoted by increasing the pressure of CO2 while crucially inhibited by phenol formed in side reaction.