Abstract: As the main protection for transformers, gas accumulation relay has problems such as protection malfunction and refusal-operation. Therefore, at present, most transformers are equipped with gas accumulation relay that only emits alarm signals without tripping. In several transformer explosion accidents occurred in recent years, the gas accumulation relay has issued alarm signals before the accident, indicating that the gas accumulation relay has certain advantages compared to other traditional protections. Therefore, the research on the characteristics of free gas inside transformers is of great significance. In order to analyze the pattern of gas movement and convergence inside oil-immersed transformer, an experimental tank with physical structure resembling that of a power transformer is set up, the corresponding two-phase flow model is established in ANSYS, and simulations of different developing stages throughout the oil-paper insulation breakdown process is conducted. Experiments confirm with simulation results, indicating that as a fault develops, gas transfers from the place where the fault occurs, to the upper surface of different structures within the tank; the gas generation rate increases throughout time, during which gas exhibits different moving patterns and undergo morphological changes. Based on the same theoretical model, simulation analysis is conducted on the internal gas behavior of the transformer, and its convergence pattern is obtained. This article provides a useful reference for the development of the novel gas protection device.