Reliability of power electronic systems is becoming more and more important in industrial activities and residential life. A good reliability design requires a concrete vulnerability analysis, which can find out the vulnerable points in the system. Increasing the reliability of the vulnerable points will significantly improve the reliability of the whole system. This paper proposes a vulnerability analysis method based on Markov process model. In this method, the components are classified into different types according to the robustness index, and then the components with high vulnerability level can be identified. A 5-level multilevel converter with self-balancing and fault-tolerant ability is taken as an example to verify the proposed method. Finally, vulnerability distribution laws of 5-level converter are summarized, and some constructive suggestions are proposed to improve the reliability of the multilevel converter.