The phenomenon of pressure rise due to arcing faults is the main cause of tank explosion. In this paper, we set up an experimental platform for arcing faults in transformer oil, and analyzed the pressure waveforms by arcing faults spectrally to study the main sources of the high-frequency, mid-frequency and low-frequency components. The main conclusions are as follows: 1) The high-frequency component in the oil pressure waveform was mainly the shock wave and its reflection wave generated during the formation of the arc channel. It was manifested as the “spike” in the early stage of the pressure waveform. 2) The mid-frequency component in the pressure waveform was mainly caused by the rapid change of bubble expansion acceleration. It was manifested as the oscillation part of the pressure waveform. 3) The pressure change caused by the volume compression of transformer oil was the main source of low-frequency component. This low-frequency component was the main component of oil pressure. The findings of this paper are an important reference for the pressure study of arcing faults and the design of tank explosion protection.