The high-energy arcing faults in power transformer can cause overpressure which would break the tank open, lead to oil-spill and even catching on fire. Facing to the great hazard of transformer overpressure during internal arcing faults duration, this paper has investigated the physical process of tank internal overpressure and revealed the relationship between the electrical faults and overpressure characteristics from the energy conversion perspective. Due to the fact that the arcing faults tests inside power transformers are extraordinarily difficult, destructive and costly, the theoretical model has been proposed and numerical simulation approaches has been employed. At first, the transformer internal arcing fault energy model, the fault pressure source and the overpressure wave propagation have been mathematically modeled considering the whole phenomenon as an acoustic problem. Then, according to the real full-scale 240MVA/220kV oil-immersed power transformer structure parameters, a transformer simulator has been set up. Thirdly, by employing 3D FEM, the transient overpressure characteristics such as pressure rise curves and effective pressure indexes at different positions have been explored. The simulation results illuminate that the chief effect to overpressure inside tank is the arc energy, the following is the positions where arcing faults happen. Additionally, both the theoretical model and the simulation method proposed in this paper can be extended to simulate the overpressure phenomenon during arcing faults in other types of oil-immersed transformers and reactors instead of costly and unpractical field tests.