The deactivation kinetics of Pd(PPh3)2Cl2 in the monocarbonylation of benzyl chloride to synthesize phenylacetic acid is studied in this paper. Solid 1-(2-pyridylazo)-2-naphthol (PAN) is used as the colouring agent, and the concentration of Pd(PPh3)2Cl2 in the system is measured through absorptiometry. The result shows that the optimum condition of the chromogenic reaction between Pd2+ and PAN is: 0.5 ml of 0.04% PAN added to 10 ml of Pd2+ solution (1.0×10-6-2.0×10-5 mol/L), and heated in a constant temperature water bath at 40℃ for about 30 min, with pH of the solution being about 3.0. The molar coefficient of absorption is 1.384×104 L/(mol·cm); the orders of the hydrolytic reaction to the concentration of Pd(PPh3)2Cl2, PPh3, phenylacetic acid and NaOH are 0.5, minus 0.8, 2 and 1.2, respectively. The activation energy (E) of the hydrolytic reaction is 75.59 kJ/mol, and the pre-exponential factor is 1.68×1012. The deactivation kinetics of Pd (PPh3) 2Cl2 in the monocarbonylation of benzyl chloride to synthesize phenylacetic acid is studied in this paper. Solid 1- (2-pyridylazo) -2-naphthol (PAN) is used as the coloring agent, and the concentration of Pd (PPh3) 2Cl2 in the system is measured through absorptiometry. The result shows that the optimum condition of the chromogenic reaction between Pd2 + and PAN is: 0.5 ml of 0.04% PAN added to 10 ml of Pd2 + solution (1.0 x 10-6 -2.0 × 10 -5 mol / L) and heated in a constant temperature water bath at 40 ° C. for about 30 min with pH of the solution being about 3.0. The molar coefficient of absorption is 1.384 × 104 L / (mol · cm); the orders of the hydrolytic reaction to the concentration of Pd (PPh3) 2Cl2, PPh3, phenylacetic acid and NaOH are 0.5, minus 0.8, 2 and 1.2, respectively. The activation energy (E) of the hydrolytic reaction is 75.59 kJ / mol, and the pre-exponential factor is 1.68 × 1012.