Abstract: A transient analysis program RETAC-PRHRS (REactor Transient Analysis Code-Passive Residual Heat Removal System) for PRHRS in AP1000 was developed using FORTRAN language. The inadvertent operation accident of the PRHRS was analyzed and transient characteristics of some main system parameters were obtained, including normalized thermal power, normalized flow rate and MDNBR in the core, system pressure, flow rate in the PRHRS. The results show that the main system parameters do not exceed specified limits and will not cause reactor shut-down. The computed results are further compared with those of thermal-hydraulic analysis codes, including LOFTRAN developed by Westinghouse and Topmeret/THEATRe developed by GSE. The trend shows a good agreement and thus proves the rationality of the modeling on PRHRS in AP1000.