In order to realize the green development of urban rail transit system, a capacity optimization scheme based on flexible DC traction power supply system is proposed. In this scheme, based on the controllable characteristics of traction network voltage of flexible DC power supply system, the photovoltaic system and energy storage system are connected to the traction network, power distribution and scheduling of traction network energy are carried out through multi-level system management. And the energy management strategy is adopted to reduce the power supply to the traction network, so as to achieve the purpose of reducing the traction capacity. In the Matlab/Simulink simulation environment, the simulation of flexible DC traction power supply system including photovoltaic system, hybrid energy storage system, station electricity consumption and locomotive load was built to verify the feasibility of the optimization scheme of traction station capacity.