Abstract: Trisodium phosphate（TSP） is an important pH buffer in the reactor safety injection system（RIS） of the HPR1000 NPP. It can significantly reduces the share of gaseous iodine released from containment by reducing the concentration of hydrogen ions in the water environment, which is of great significance to avoid radionuclide leaks and to ensure the safety of nuclear power plant operators and the surrounding environment. This study aims to explore the dissolution characteristics of trisodium phosphate along with the change of parameters such as temperature, flow and effective contact area. Therefore, a high-temperature airtight circuit was built to simulate the variety of dissolving process of TSP in the containment vessel, and the least square method was used to analyze the experimental data. The study results can provide important reference for design calculation and safety analysis of HPR1000.