Hydrocarboxylation of styrene catalyzed by water-soluble Pd-TPPTS complexes was investigated. The reaction conditions, including reaction pressure, temperature, time and etc. have a significant influence on the catalytic performance. It was found that the yield of total acids and the selectivity towards 3-phenylpropionic acid were enhanced by adding a suitable amount of alkali metal halide salts. In addition, the reaction mechanism and the role of alkali metal halide salts in the reaction were discussed on the basis of the characterization of H-1 NMR and P-31 NMR.