Abstract: The RISMC methodology is an advanced safety analysis method in nuclear power plant. It aims at coupling of the DSA method and the PSA method. In this paper, a typical small break LOCA of a passive nuclear power plant is taken as the case of study, expecting to get more realistic risk insights using RISMC methods. Based on the theory of the Risk Informed Safety Margin Characterization methodology, the development process of the accident is studied. The specific sequence is selected to be studied. The important systems are discretely branched（such as the Automatic Depressurization System）, and the important uncertainty parameters are sampled（such as the ADS valve and the IRWST valve resistance coefficient）. The event tree is re-established, and the success criterion of the head is modified. The fault tree is re-established. After that the uncertainty parameters are sampled for specific sequences and the accident simulation of the whole plant is carried out for each condition. Thus, the frequency of each sequence and the conditional failure probability under the specific condition are obtained, and the core damage frequency（CDF）value based on the RISMC method is finally calculated. Through the analysis and calculation of the RISMC method, it is concluded that the calculated value of the CDFin each selected sequence is reduced to a certain extent. This paper verifies the feasibility of the RISMC method in application on passive nuclear power plants, and also proves that the method can remove unnecessary conservatism. More realistic assessment of the risk helps us quantify the safety margin of nuclear power plants better.