Differential testing arrangement in DC power supply system return circuit of transformer substation belongs to electric power power supply unit technical field, for the problem of solving the prior art existence, and the direct current fuse load end of direct current output interface connection final stage, the inverter that is incorporated into the power networks is sent to through current sampling circuit and voltage sampling circuit's sampling to the direct current output interface, the inverter that is incorporated into the power networks converts direct current to the alternating current that can be incorporated into the power networks and transmits for inserting the electric wire netting unit, through inserting the electric wire netting unit alternating current signal conveying telegram in reply net, different current output is adjusted by computer control to the inverter that is incorporated into the power networks, realizes that the 0 -500A electric current makes the protecting component action in DC power supply return circuit, the AD converter is given signal transmission simultaneously to current value and magnitude of voltage when current sampling circuit and voltage sampling circuit just can sample the circuit breaker action, analog signal is changed to the AD converter, converts digital signal transmission to and gives the computer, the computer calculates data signal, the operating characteristic that judges the direct current return circuit according to the electric current and the operate time of difference, the computer passes through the statement crossover tool signal input statement output unit.