The invention discloses a paying-off-free testing system for parameters of a transformer station grounding device, and the system comprises a first phase line and a second phase line which are selected from a low-voltage outgoing wire which is not usually put in operation or can regularly withdraw from operation at a transformer station, a current electrode grounding knife switch which is disposed on the first phase line and is near the far end of a to-be-detected transformer station, a voltage electrode grounding knife switch which is disposed on the second phase line and is near the far end of the to-be-detected transformer station, and transformer station internal testing equipment for controlling the switching of all switches. According to the invention, the low-voltage outgoing wire which is not usually put in operation or can regularly withdraw from operation at the transformer station is selected, and the A and C phases of the outgoing lines are independently defined as a voltage line and a current line for testing. The positions of the voltage and current electrodes of the A and C phases of the low-voltage outgoing wire are selected according to the size of the transformer station grounding device, so as to achieve a test loop. The system saves a large amount of manual operation in testing, guarantees that a test result is not affected by the quality of a tester and the external environmental factors, and improves the accuracy of the test result.