Abstract: In order to conduct the integrated simulation and verification for the matching of the first-second-third loops of HPR1000, and to accurately evaluate the actual state and performance of the plant, this study builds a simulation platform based on RELAP5/Simulink software. A high-precision integrated model of the first, second, and third loops of the Fangchenggang Unit 3 HPR1000 demonstration project is established, and simulation analysis is conducted for selected transient conditions. The results verify the safety, reliability, and control performance of the control system of the HPR1000 under the aforementioned conditions, and demonstrate the feasibility of the RELAP5/Simulink software coupling technique.