C6F12O (1,1,1,2,2,4,5,5,5-Nonafluoro-4-(trifluoromethyl)-3-pentanone) is an environmental friendly gas insulating medium with the potential to replace the strong greenhouse gas SF6 for medium and low pressure gas insulated equipment. In this paper, the decomposition mechanism of C6F12O-CO2 gas mixture under different temperature and pressure conditions were explored based on the ReaxFF molecular dynamics. The thermal decomposition characteristics of C6F12O-CO2 gas mixture were also tested. We found that the main decomposition particles of C6F12O-CO2 gas mixture incudes CF3, CF2, CF, C3F7, CO, COF2, C5F9O and F, and the content of CF3, CF2, CF, C3F7, CO and F is high. The thermal decomposition of C6F12O-CO2 gas mixture mainly produces CF4, C2F6, C3F8, C3F6 and C5F12, among which the content of C2F6 and C3F6 are the highest, followed by C3F8 and the content of CF4 is the lowest. Relevant results not only reveal the decomposition mechanism of C6F12O-CO2 gas mixture comprehensively, but also provide an important reference for exploring the decomposition characteristics and engineering applications of C6F12O-CO2 gas mixture.