The elastic electrical contact components in the valve-side ascending flanged base of the converter transformer are prone to overheating, leading to the thermal decomposition of transformer oil and the generation of gases such as CH4, C2H4, C2H6, C2H2, and H2. Due to the complexity of the ascending flanged base structure, the transformer oil is difficult to participate in the main oil circulation, and characteristic gases take a long time to diffuse to the oil chromatographic sampling point, making it difficult to detect faults in a timely manner. Therefore, it is necessary to study the diffusion distribution characteristics of characteristic gases in the valve-side ascending flanged base of the converter transformer. By using finite element analysis and multi-physics field coupling methods, a thermal-mass transfer model of characteristic gas diffusion in the valve-side ascending flanged base of the converter transformer is established, and the velocity field and characteristic gas content distribution law of the ascending flanged base are obtained. The research results show that when gas production reaches three months, the highest CH4 concentration in the gas generation position is 83.74 μL/L, and the total hydrocarbon concentration at the top of the ascending flanged base is 27.86 μL/L, which is much lower than the standard attention value of characteristic gases, potentially leading to failure in fault detection.