Abstract: Air bubbles are a typical defect of oil-paper insulation system, which can significantly reduce the insulation performance of transformer oil. In order to clarify the hazard of bubbles to the oil-paper insulation system, this paper builds a bubble dynamics observation platform to study the deformation characteristics and the influence law of bubbles adsorbed on the electrode surface under AC electric field. It is found that the adherent bubble in transformer oil stretches in the direction of the electric field and pulsates at a frequency of 100 Hz. The degree of bubble deformation increases gradually with the increase of electric field strength, bubble radius, and temperature. A simulation model of gas-liquid two-phase flow is built based on the phase field method, and the simulation and experimental results are verified with each other, revealing that the essence of bubble deformation is the balance of electric field force and surface tension performance. The bubble affects the electric field distribution of the oil gap, and its internal field strength decreases with the stretching of the bubble along the electric field direction. The research in this paper can provide a reference for in-depth evaluation of the hazard of bubbles to liquid insulation and design of transformer insulation.