C4F7N　is　one　of　the　promising　candidates　to　replace　SF6　as　insulating　medium　in　power　equipment.　Meanwhile,　the　effect　of　an　O　atom　on　C4F7N　decomposition　is　still　unclear　but　it　greatly　influences　the　decomposition　mechanism　and　insulating　properties　of　C4F7N.　In　this　paper,　the　B3LYP　method,　in　conjunction　with　6-311G(d,p)　basis　set,　is　used　to　study　the　decomposition　mechanism　of　C4F7N　with　an　O　atom.　The　molecular　structures　of　reactants,　products　and　transition　states　(TS)　in　the　C4F7N　decomposition　pathway,　which　consists　of　25　reactions,　are　optimized.　The　potential　energy　surface　of　each　reaction　and　the　vibrational　frequencies　of　each　molecule　or　TS　are　studied　with　same　method　as　the　prerequisite　for　rate　constant　calculations　using　transition　state　theory.　The　results　show　that　the　CN　bond　breaks　with　the　highest　energy　of　321.91　kcal　mol-1,　so　reaction　(R21)　(C4F7NO　→　C4F7O　+　N)　plays　a　less　important　role　than　(R2)　(C4F7NO　→　C3F4NO　+　CF3),　(R5)　(C4F7NO　→　C3F7O　+　CN),　(R11)　(C4F7NO　→　TS1　→　C3F5N　+　CF2O)　and　(R15)　(C4F7NO　→　TS2　→　C3F3NO　+　CF4)　in　the　decomposition　of　C4F7NO.　CF2O　and　CF4,　characteristic　decomposition　products　of　C4F7N　+　O　which　have　negative　impacts　on　the　insulating　gas,　are　generated　in　reactions　(R11)　and　(R15)　with　a　higher　rate　constant　at　1500　K　above,　respectively.　CF3　and　F3C-O　are　important　in　C4F7N　with　O　decomposition　because　of　the　high　overall-generation　rate　constants.　This　work　is　expected　to　provide　a　theoretical　basis　to　evaluate　the　insulation　performance　of　C4F7N-insulated　power　equipment　and　develop　novel　gas　sensors　for　insulating　condition　monitoring.　©　2019　IOP　Publishing　Ltd.