Transformer plays an important role in UHV transmission system. The overpressure caused by arcing fault inside transformer deforms the oil tank, breaks the tank open and makes it possible for transformer explosion accident, which seriously threatens the safety and stability of the power grid. Therefore, research on pressure generation caused by large-current arcing faults and explosion-proof design of high-voltage transformer is of great significance to the proper operation of power grid. Herein, the physical process of pressure generation caused by transformer internal arcing fault has been focused on, and the relationship between the characteristics of arc and overpressure has been revealed.Since the internal arcing fault tests are destructive and costly, this study has taken the approach of numerical simulation and modeled the process of pressure generation by arc. The pressure generation model was based on the large-current arc model, gas production model and gas-liquid pressure relationship. This study established a bridge from arc discharge to pressure generation through the conversion of internal energy. Finally, current and length of internal arcing fault were systematically investigated to figure out the mechanism of arc on pressure generation.By compared with the results of transformer internal arcing fault test in published literature, the model and simulation results were proved correct. The results of simulation show that the length of internal arc maintains a greater influence on the overpressure than current, which is caused by the length of internal arcing fault can directly increase gas production efficiency and energy conversion efficiency.This pressure generation model in this paper is based on the internal energy conversion, providing an approach to simulate the pressure distribution inside transformer when arcing fault occurs and showing that the reduction of arc length can be the key to decrease the pressure inside oil tank, which contributes to the explosion-proof design of high voltage transformer.