In order to find out the detection effectiveness of GIS insulation defects when suffering different overvoltages, a coaxial bus line of 220kV with a needle attached to it was established as an experimental sample to simulate the practical condition that there existed a metallic particle on the bus line of real GIS. Research was carried on with the help of 1MV steep-fronted impulse generator. By changing the wave front resistance of Marx generator, different kinds of double exponential impulse voltages could be produced. Influence of wave front time (tf) on 50% discharge voltage (U50) and voltage-time (v-t) characteristic of GIS was studied in this article. Results indicate that with tf increasing, U50 presents an increasing tendency overall; with pressure (P) increasing, when tf is short, U50 hardly changes with P, but when tf becomes long enough, U50 changing with P appears as a hump curve. As for v-t characteristic, it turns out that, with the length (l) of needle decreasing, both breakdown voltage and its dispersibility increase; it’s also found that the shorter the needle is, the shorter delay that turning point occurs becomes. When tf changes, shape of v-t characteristic changes too, the v-t characteristic integration of different wave front times looks like a concave basin. The above phenomena are explained in the view of the ionization, diffusion and recombination of space charges.