The B3 LYP method based on density functional theory at the level of lanl2 dz was used to investigate the thermodynamic properties of the carboxylation reaction from phenylpentacarboxylic acid to mellitic acid with cesium carbonate as the catalyst. This reaction is spontaneous and exothermic in thermodynamics. The reaction equilibrium constant decreases with increasing temperature. The effects of mixing mode, temperature, pressure, and time on the carboxylation reaction were examined by single-factor experiments. The results show that the hydrolysis of phenylpentacarboxylic acid and cesium carbonate to form salts is beneficial to the subsequent reaction. In the same reaction time, temperature has a great influence on the carboxylation reaction. When the required temperature is not reached, the reaction cannot proceed even under a certain pressure. In addition, the yield and selectivity of mellitic acid increases first and then decreases with time.