To address power quality and system stability issues caused by strong randomness and fluctuations on both supply and demand sides due to the large-scale integration of renewable energy into railway traction power supply systems, a coordinated control strategy for off-grid renewable energy railway traction power supply systems is proposed. First, the topology of the traction power supply system is constructed, and models for photovoltaic (PV) generation, energy storage, and traction conversion systems are established. Second, system operation modes are categorized based on the power imbalance between PV generation and loads, train operating power, and the state of charge (SOC) of energy storage to achieve rational energy allocation. Then, a coordinated control strategy is designed to address DC bus voltage instability resulting from source-load power imbalance when energy storage power reaches its limits. Finally, hardware-in-the-loop experiments demonstrate that the proposed strategy effectively enhances the system’s resilience to power fluctuations from both source and load sides, mitigates energy storage overcharging and over-discharging issues, and resolves DC bus voltage overlimit problems.