Abstract: The turbine protection system of a nuclear power plant is designed to ensure the safe shutdown of the turbogenerator and mitigate or limit accidents under predefined fault conditions. During the implementation of the main steam valve trip test at a nuclear power plant, two consecutive fractures of the main steam valve stem occurred, severely hindering the turbine startup process. To identify the underlying mechanism, a comprehensive root cause analysis was conducted. In this study, the potential factors contributing to the fractures were systematically examined, including the mechanical properties, chemical composition, grain size, non-metallic inclusions, and metallographic structure of the valve stem material, as well as the functionality of the hydraulic actuator drain buffer. The results indicate that the failure of the hydraulic actuator—specifically the loss of the drain buffer function caused by manufacturing deviation—serves as the main reason. Concurrently, inherent material deficiencies, such as a low grain size rating, an abundance of non-metallic inclusions leading to reduced impact toughness, and stress concentration at the stem root, collectively precipitated the brittle fracture. Based on the above results, reducing the oil discharge clearance of the actuator and selecting valve stems with qualified materials can effectively solve the problem of valve stem fracture during the tripping process of the main steam valve, while also providing guidance and reference for other units to address similar issues.