To obtain impulse impedance characteristics for grounding device of transmission tower, a small and portable instrument is developed. It can stable output the 8/20 µs and 10/350 µs standard lightning currents with simultaneous measurement of the response transient voltage waveshapes, which simulates the currents of direct and indirect lightning strikes, respectively. Tests were made to an standard resistance, the grounding grids of the laboratory and a 10kV transmission tower and a 10kV transmission tower with a transformer on site, while the results show that the instrument is stable, the testing method is simple and the test results can accurately reflect the basic physical properties of the grounding device of 10kV transmission tower.