AbstractEngineering experience shows that for transformers with long service life and serious moisture exposure, the inter‐turn paper insulation inside the winding will precipitate bubbles under the condition of electric field and conductor heating, which will endanger the equipment insulation. In order to explore the influence factors of bubbles formation in the moist insulating paper of transformers, based on the existing research on the influence of moisture content and gas dissolution in transformer oil, this study further considers the influence of air pressure (AP) and electric field and establishes an experimental platform to change the air pressure and electric field, thereby studying the formation characteristics of bubbles in oil‐paper insulation. The results show that air pressure and electric field can affect the initial temperature, volume, and shape of bubble formation. From 0 to −0.05 MPa for air pressure, the initial temperature of bubble generation decreased by 41°C, its maximum two‐dimensional projected area increased by 418.4% with more severe distortion, and the breakdown voltage decreased. When the AC voltage applied on the needle plate electrode increased from 0 to 10 kV, the initial temperature of bubble formation increased by 29°C, and the maximum two‐dimensional projection area increased by 336.6%.