Abstract: At present, the relevant studies indicate that adding metal passivators in oil-immersed power transformer would result in the generation abnormal of dissolved gases in oil. In order to understand the degradation and gas generation prcoess of benzotriazole and its derivative passivators in oil-paper insulation and guide fault diagnosis of transformer, the benzotriazole (BTA) that widely used in the oil-paper insulation was selected as the research object, the molecular model system of BTA was established in the molecular simulation environment, and the molecular dynamics simulation was conducted to analyze the pyrolysis prcoess of BTA molecule from 2000 K to 3000 K based on reactive force field (ReaxFF). Combined with the thermogravimetric analysis of BTA and the accelerated aging experiment under thermal field condition, the mechanism of BTA degradation and gas generation in oil paper insulation was analyzed from micro and macro levels. The simulation results show that BTA can be pyrolyzed to form small molecular hydrcoarbon free radical, small molecular hydrcoarbon compound, H· and N·free radicals, which leads to the formation of low molecular hydrcoarbon gases, H₂, N₂ and NH₃. The experiment results show that BTA is pyrolyzed continuously as the temperature increase, and the addition of BTA in the oil lead to the generation abnormal of dissolved gases in oil. The content of H₂, CO and CO₂ increase significantly, while the total hydrcoarbon content decreases. Finally, the mechanism of BTA degradation and gas generation in oil-paper insulation under thermal field condition was deeply investigated by analyzing the molecular simulation results, the generation of gas dissolved in oil and the variation trend of physical and chemical parameters of oil.