Abstract: The sample of tiopronin tablets was ground, dissolved and diluted with water to prepare the sample solution for test. 2.00 mL of Pd²⁺ standard solution, 4.00 mL of the sample solution for test and 1.00 mL of acetic acid-sodium acetate buffer (pH 5.0) were successively added into a 10 mL colorimetric tube, and the mixture was made its volume with water, shaken well and settled for 5-10 min. The tiopronin standard solution was used as reference, and the absorbance of the system was determined at 345 nm. It was shown that tiopronin and Pd²⁺ formed a complex at molar ratio of 2:1 in a weak acidic environment. Linear relationship between mass concentration of tiopronin and its absorbance was kept in the range of 0.20-60.00 mg·L^-1, with detection limit (3s/k) of 0.003 5 mg·L^-1. Recovery test was made on the actual samples, giving results of in the range of 98.0%-103%, and RSDs (n=5) of the determined values were ranged from 0.83% to 3.5%. This method was used for analysis the actual samples, and the measured values were basically consistent with the labeled amounts.