C6F12O (1,1,1,2,2,4,5,5,5-nonafluoro-4-(trifluoromethyl)pentan-3-one) is one of the most potential alternative insulation medium to SF(6)due to its excellent electrical strength and environmental effect. Although several works on the insulation and decomposition properties of C6F12O have been carried out in recent years, the detailed decomposition pathways and mechanism of C6F12O haven't been revealed up to now. A comprehensive analysis of the decomposition mechanism of C6F12O is performed through high-level quantum chemistry calculations with DFT and TST in this paper. The results show that more than ten decomposition products are formed. C4F10(a), C5F12(b), and CO can be used for evaluating slight failures. C3F8, C2F6, C2F4, C3F6, C4F10(b), C5F12(a), C(6)F(14(a))and C(6)F(14(b))etc. can be taken as typical products when a general failure is caused in the equipment. CF(4)can be used as a basis for determining whether a serious failure has occurred in the gas-insulated electrical equipment. According to the analysis, primary decomposition products such as C(4)F(10(a))and C(5)F(12(b))have relatively high dielectric strength. Besides, the concentration of decomposition products of C6F12O is low to have a major impact on the overall electrical performance at normal conditions. However, the insulation properties of C6F12O gas mixtures after the prolonged operation or multiple arc extinctions deserve further investigation. In the view of human safety and environmental effect, several toxic products such as C2F4, CO, and C(3)F(6)should be effectively managed and handled with care. This paper could provide a theoretical basis for related engineering applications.