There is increasing interest in small modular reactors (SMRs), where the severe accidents management should be taken into account in the design of SMRs. A series of systems are designed to mitigate the severe accidents, including the severe accident relief valves (SADV), in-vessel retention systems (IVR), containment pressure suppression systems (CPS), containment heat removal systems (CHR), and containment filtration and venting systems (CHE). This paper focuses on the analysis of the activation time intervals for the mitigation systems, and some improvements of the severe accident management are proposed on the basis of the results. The paper introduces the design features, highlighting their implications for severe accident phenomena, and presents a comprehensive overview of the mitigation measures for typical severe accidents in SMRs, including the control of the primary circuit pressure and the containment pressure, and the reduction of the radioactive releases. To assess the effectiveness of these mitigation measures and investigate the activation time under severe accidents, a combination of probabilistic, deterministic, and engineering experience-based methods is used to select typical severe accident sequences. Systematic analysis procedures are then conducted to calculate and analyze the operational effectiveness of each mitigation system. The results show that the effective operational time interval for the various mitigation systems makes valuable insights into their performance during severe accidents. Based on the results, some recommendations for target severe accident management are put forward, including the startup conditions for each mitigation system, the selection of status parameters, the determination of activation time, and other relevant factors, within the overall severe accident management strategy, which are contributed to improve the safety and reliability of SMRs, providing some useful references for accident prevention and response in actual operation.