Abstract: To study the evolutionary law of pressure wave at the initial stage of arc discharge, the paper conducts discharge experiments in transformer oil. Schlieren images are collected to observe the development process of bubbles and pressure waves at the initial stage of arc discharge. The arc voltage, current, pressure and wall vibration acceleration are measured. The characteristic changes of arc under different electrode gap distances are analyzed. An energy transfer path model of arc discharge in oil is established, and the evolution law of energy transfer ratio is obtained. The experimental results indicate that the occurrence of multiple breakdown processes between the electrodes leads to the generation of pressure waves accompanying the appearance of pulsed voltage and current. In the discharge process with electrode gaps of 1, 2 and 3 mm, pressure waves can be observed 2, 5 and 7 times, respectively. The voltage, current, energy, pressure wave number and intensity increase with the increase of arc energy. The maximum transfer ratios of arc energy to internal energy, mechanical energy, thermal conduction and radiation energy during the discharge process are 29.35%, 15.99%, and 6.52%, respectively. The research results have preliminarily elucidated the pressure wave characteristics and parameter variations of early-stage arc in transformer oil under AC voltage.