The voltage waveform on the insulator by a direct lightning stroke is a short-tail lightning impulse. Experimental study and analysis of insulator breakdown characteristics under short-tail lightning impulses are given in this paper with the contrast of standard lightning impulse. The impacts of crossarm and transmission lines are also considered. The experimental results indicate that the insulator 50% breakdown voltage under short-tail impulse is significantly higher than that of standard wave with both positive and negative polarities, about 25~30%. The 50% breakdown voltage of insulator with negative-polarity short-tail impulse is about 5% higher than with positive-polarity one. The insulator voltage-time characteristics are also given with short-tail and standard lightning impulses.