Abstract: Based on the structural and operational characteristics of AP1000, a thermal-hydraulic computer code RETAC was developed using FORTRAN language. The code was adopted to analyze the inadvertent opening of automatic depressurization system (ADS) and transient characteristics of some main system parameters were obtained, including pressurizer pressure, normalized core thermal power, normalized core flow rate, core average temperature, the maximum fuel temperature and MDNBR. The results show that the maximum fuel temperature and MDNBR do not exceed specified limits and meet the safety criteria under the protection of pressurizer low-pressure shut-down signal. The computed results were compared with the results of thermal-hydraulic analysis code LOFTRAN for AP1000 developed by Westinghouse Electric Corporation. The trend shows a good agreement and thus proves the applicability and accuracy of the modeling of RETAC and the calculation of ADS critical flow rate.