Abstract: Thermal stress is one of the main factors leading to the deterioration of the oil paper insulation material of traction transformer, and also affects the safe and stable operation of traction transformer and the whole EMU train. No. 45 insulating oil and Nomex T410 insulating paper are taken as test objects. Based on the actual operating conditions of the vehicle traction transformer, the accelerated thermal aging test is firstly carried out to test the breakdown voltage, dielectric loss factor, moisture content and dissolved gas in the oil in different aging cycles. It is found that the insulation performance of the material decreases with the aging time. The insulation oil deteriorates completely around the 20th aging cycle, and the test value exceeds the index. Secondly, the ReaxFF reaction molecular dynamics simulation is carried out to reveal the mechanism and path of the generation of small and medium molecular substances in the process of thermal aging of oil-paper insulation, which provides theoretical support and reference for the analysis method of dissolved gas in oil of this kind of transformer from the microscopic level. The results show that the pyrolysis products of the oil-paper mixed insulation system include H₂, CH₄, C₂H₆, C₂H₄, CO, CO₂, H₂O and CO₂H₂.