The excavation process of shield tunnel involves many unsafe actions, which inevitably affect the safety of existing railway subgrades. Traditional safety analysis methods, such as Fault Tree Analysis (FTA), mainly assume that the probabilities of different factors are independent and it can’t fully consider accidents caused by the interaction among these factors. In order to effectively assess the safety risk of shield tunnel under existing railways, this study applies the Systems-Theoretic Process Analysis (STPA) method to shield tunneling projects under existing railway subgrade. The safety issues in shield tunneling project under existing railways are treated as control problems. Taking an actual project as an example, the unsafe control actions during the construction process are identified, and the causal factors leading to these unsafe control actions are analyzed. Finally, a numerical simulation model of shield tunneling undercrossing existing railways is established to conduct a quantitative safety analysis of typical unsafe control actions. The results indicate that the STPA method can effectively analyze the safety of shield tunneling undercrossing existing railways and reveal risks that traditional methods may not discover.