The invention discloses a transient over-voltage suppression method for the on-site impulse test on an ultra-high-voltage GIS. According to the technical scheme of the method, one side of a power supply side casing is connected with a compact surge voltage generator via a high-voltage anti-corona conductor. The compact surge voltage generator is used for generating lightning waves of specified amplitude required by the on-site impulse test. The top of a no-load casing is connected with a transient over-voltage suppressor via another high-voltage anti-corona conductor. The transient over-voltage suppressor comprises a plurality of unit modules and transformer oil, wherein the plurality of unit modules are connected in series via introduced copper bars. During the occurrence of a fault, the amplitude of the transient over-voltage is ensured to be smaller than the amplitude of the highest impulse voltage allowed by the on-site impulse test on the ultra-high-voltage GIS by means of the transient over-voltage suppressor. Therefore, on the condition that no breakdown occurs, the waveform of the test voltage inside the GIS is substantially the same with the voltage waveform thereof when the GIS is not accessed to the transient over-voltage suppressor.