Abstract: This study established and fully validated a reliable high performance liquid chromatography method to determine radiochemical purity and iodide content in diagnostic sodium iodide¹³¹I capsules. In this established analytical method, a chromatographic column filled with octadecylsilyl-bonded silica gel was employed. The mobile phase was prepared by mixing sodium chloride-n-octylamine solution with acetonitrile at a volume ratio of 20∶1. A radioactivity flow detector connected in series with an ultraviolet spectrophotometer served as the dual detection system, which allows the simultaneous quantification of radiochemical purity and iodide content of diagnostic sodium iodide¹³¹I capsules. Corresponding validation results for iodide determination indicate that excipient solution, purified water and mobile phase exert no obvious interference at the characteristic retention time of the iodide peak. For the determination of iodide content, iodide exhibites an excellent linear relationship within the concentration range of 0.01 to 40.00 mg/L, with a correlation coefficient(r) of 1.000. The limit of detection(DL) and limit of quantitation(QL) for iodide are determined to be 0.002 mg/L and 0.01 mg/L, respectively. For the determination of radiochemical purity, iodide¹³¹I ions present favorable linearity over the radioactivity concentration range from 7.696 MBq/L to 575.350 MBq/L, and the correlation coefficient(r) is also 1.000. The DL and QL of iodide¹³¹I ions are 1.517 MBq/L and 5.106 MBq/L correspondingly. Moreover, when chromatographic conditions including column temperature, flow rate, mobile phase composition and pH value were altered, the resolution between the peaks of iodide¹³¹I ions and iodate¹³¹I ions in the test solution consistently meet the regulatory requirements. The accuracy and precision of the analytical method for iodide¹³¹I ions complies with all method validation specifications. The established HPLC method for determining the radiochemical purity and iodide content of diagnostic sodium iodide¹³¹I capsules complies with the requirements for analytical method validation in terms of specificity, accuracy, precision, limit of detection, limit of quantitation, linearity, linear range and robustness. This study establishes a high performance liquid chromatography method with two detectors connected in series, which overcomes the technical limitation of the paper chromatography specified in the Chinese pharmacopoeia that cannot simultaneously determine the radiochemical purity and iodide content of diagnostic sodium iodide¹³¹I capsules. By optimizing the pretreatment method for enriching radioactive components in low-activity samples and chromatographic conditions, this approach addresses the low detection sensitivity of the high-performance liquid chromatography method adopted in the European pharmacopoeia. The established method features favorable resolution and high sensitivity, enabling simultaneous quality control of the two key indicators. It further improves the quality evaluation system for diagnostic sodium iodide¹³¹I capsules and provides a reliable technical basis for the standardized application of quality control for this radiopharmaceutical.