Abstract: Distribution transformers suffer from intrusion of nanosecond electrical stress in the early conduction environment of high-altitude electromagnetic pulse (HEMP), which is likely to cause insulation failure or damage at a solder joint of winding. In this paper, the V-t characteristics, breakdown probability, the relationship between pulse voltage amplitude and pulse cumulative times during dielectric breakdown (U-N characteristics) of oil-impregnated paper were studied, with a tip defect under nanosecond pulse voltage. The law and mechanism of paper damage under nanosecond pulse voltage were analyzed based on discharge characteristics, microscopic morphology and streamer simulation. The results show that V-t characteristics, breakdown probability distribution, U-N characteristics have an obvious polar effect, which is related to the interface barrier charge; the pulse-induced damage to oil-impregnated paper can be divided into mild grade and severe grade. Mild damage is prone to form surface spot defects, showing a partial discharge mode with high repetition and low amplitude; severe damage is easy to form cracking defects, showing a partial discharge mode with low repetition and high amplitude. The difference in damage is related to the characteristics of surface streamer. This paper can provide references for improving the survival ability and evaluation techniques for distribution transformers in HEMP environment.