The power supply capability serves as a pivotal element in the operational evaluation of subway traction power supply system. The results can offer critical insights for the formulation of preliminary operational strategies and train scheduling plans. This study, grounded in the national standards of the subway industry, identifies key indicators for the evaluation of power supply capability and subsequently designs a comprehensive evaluation framework for the power supply capability of subway traction power supply system, considering the boundaries of each indicator. Moreover, by integrating the power supply capability of the subway traction power supply system with the train departure intervals, a robust method for evaluating power supply capability and a procedure for computing the maximum power supply capability are established. This facilitates the real-time evaluation of power supply capability under various operational plans of the subway traction power supply system and the determination of the maximum power supply capability. The method proposed in this paper can visually and accurately display the situation of the system, providing a reference for the evaluation of subway operation.