Abstract: There are many differences during the severe accident process between the small modular reactor (SMR) and the traditional large-scale pressurized water reactor, because of the differences in structure and design issues for both reactors. Therefore, the severe accident analysis model was established based on the design characteristics of the SMR. Then, the severe accident process, the thermal-hydraulic phenomena and the system safety were researched. Based on the results of the severe accident process for SMR, the optimization and improvement of the automatic depressurization system (ADS) were proposed. The results of this study are the necessary foundations for the development of SMR severe accident prevention and mitigation measure.