The C4F7N/CO2/O2 gas mixture is the most promising eco-friendly gas insulation medium available. However, there are few studies on the mechanism of the influence of the buffer gas O2 ratio and its role in the decomposition characteristics of C4F7N/CO2. In this paper, based on the ReaxFF reaction molecular dynamics method and density functional theory, a simulation of the thermal decomposition process of the C4F7N/CO2 mixture under different O2 ratios was carried out at temperatures in the range 2000-3000 K. A constructed model of the C4F7N/CO2/O2 mixture reaction system was used that included the possible reaction paths, product distribution characteristics and their generation rates. The calculation results show that the thermal decomposition of C4F7N/CO2/O2 mainly generates species such as CF3, CF2, CF, F, C2F5, C2F4, C2F2, C3F7, C2F2N, C3F4N, CFN, CN, CO, O, and C. Among them, the two particles CF2 and CN are the most abundant. The first decomposition time of C4F7N is advanced by the addition of O2, while the amount of C4F7N decomposed and the generation of major decomposed particles decreases. The addition of 0%-4% of O2 decreases the reaction rate of the main decomposition reaction in the reaction system. Quantum chemical calculations show that the dissociation process occurring from the combination of C4F7N with O atom is more likely to occur compared to the direct dissociation process of C4F7N molecules. The conclusions of this study provide a theoretical basis for the optimization of the application ratio of C4F7N/CO2/O2 and the diagnosis of its equipment operation and maintenance.