Taking a certain subway line as an engineering example, this paper first introduces the power supply scheme of the general traction rectifier unit, and then proposes and expounds a flexible DC traction power supply system that uses a bidirectional converter device to replace the general rectifier unit. By in-depth analyzing the specific impacts of this new system on the setting of traction substations, the configuration of external power sources, the control of rail potential, a significant saving in the overall investment of the power supply system is finally achieved. The relevant research results can provide useful references for the design of power supply schemes for subsequent similar subway projects.