Abstract: To investigate the severe discharge characteristics of highly non-uniform electric fields in practical large power transformer environments, a 110kV real transformer testing platform is built. The discharge process of needle plate electrodes is monitored and the discharge phenomena are classified. The pulse current characteristics of the oil gap penetration discharge are summarized The evolution process of oil gap penetrating discharge channels under different polaritiesis is explored by combining the characteristics of ultra high frequency. The research show that there is a phenomenon of oil gap breakdown in the cardboard barrier, which is an important manifestation of severe discharge in the oil-paper insulation structure. There are significant differences in the shape of the oil gap penetrating discharge with different polarities, and it exhibits cluster-like pulse currents with different pulse sequences of extremely non-uniform electric field. The evolution process of the oil gap penetrating discharge channel is similar to the development process of long air gap discharge. The evolution process of discharge channels under positive polarity can be divided into two stages: the initial corona transition to discharge channel and the continuous discharge channel development. The phenomenon of discharge channel extinction may occur during the development of continuous discharge channels. The evolution process of discharge channels under negative polarity can be divided into two stages: the initial corona and the connection process. There are spatial step discharges with unequal time intervals during the connection process. This paper can provide reference for the discharge characteristic test and physical model research of large power transformers.