The invention discloses a closed-loop testing system of a direct-current transformer. A standard source collection module and an optical fiber receiving module are arranged in the closed-loop testingsystem, analog signals outputted by a standard source through a secondary terminal are acquired through the standard source collection module and converted into first digital signals, and second digital signals outputted by the direct-current transformer applied with standard source primary terminal output signals are acquired through the optical fiber receiving module. Under the action of synchronizing clock signals transmitted by a synchronizing module, the first digital signals and the second digital signals are synchronously received, processed, packed and uploaded to an upper computer through a digital synchronization processing module connected with the synchronizing module, and finally, the upper computer completes closed-loop testing of the direct-current transformer according to the first digital signals and the second digital signals. The closed-loop testing system lowers requirements on stability of a standard source and improves accuracy of testing results.