With the integration of new energy, the uncertainty of new energy output will have a serious impact on the operational economy and reliability of the traction power supply system. In order to cope with prolonged extreme weather, achieve long-term stable energy supply for the system, and improve energy utilization efficiency, this paper proposes a capacity allocation method for traction power supply system considering wind-photovoltaic-hydrogen-storage access under extreme weather conditions. Firstly, a capacity allocation model was established to minimize the annual investment cost, operating cost, and carbon trading cost of the system. Then, extreme weather scenarios are obtained through the analysis of extreme power data measurement indicators. Finally, the effectiveness of the relevant schemes was tested through numerical simulations, and the results indicate that the proposed capacity allocation method has improved the consumption capacity of renewable energy, enhanced the economic efficiency of the system, and promoted the green development of rail transit.