The bi-directional converter device is increasingly applied in the newly built urban rail transit lines in various places due to its remarkable advantages in improving the voltage and reducing the losses of the traction power supply network, meanwhile better utilizing the train regenerative braking energy. This paper analyses the over-current characteristics of different power electronic elements and the external characteristics of the bi-directional converter device, then analyses the impacts on the urban rail transit traction power supply system during DC short-circuit faults after the bi-directional converter device is applied, and provides conclusions and suggestions from the perspective of the rail transit power supply system designer, such as IGBT over-current capability is relatively weak compared with diodes and thyristors, short-circuit may cause multiple bi-directional converters to fail, and it is necessary to simulate and study the distribution of short circuit current during a power supply system design, etc. therefore provides solutions and methods for designing a safer, more reasonable, more reliable urban rail transit traction power supply system.