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Investigations into alternative gases to reduce the usage of SF6 have great benefits on global warming issues and the health of maintenance personnel. C4F7N is one of the most remarkable replacements for SF6 owing to its good insulating performance, low global warming potential and non-toxicity. The decomposition mechanism of C4F7N is important in evaluating the insulation performance but it is still not clear. Therefore, the B3INP/6-311G(d,p) method in conjunction with transition state theory is used to study the decomposition mechanism of C4F7N. Sixteen reactions are found in the decomposition pathways of C4F7N. The optimized configurations and harmonic vibrational frequencies of selected species are very consistent with experimental data to verify the method adopted in this paper. The potential energy surface of these reactions are obtained and the reaction mechanisms are analyzed. The rate constants over 300 K-3500 K relevant to the insulation breakdown temperature are computed based on the above quantum chemistry calculations and dominant reactions in different temperature regions are selected. For example, reaction R5 (C4F7N -> TS2 -> FCN + CF2CFCF3) is the most important reaction leading to the dissociation of C4F7N below 600K, while reaction R2 (C4F7N -> C2F4CN + CF3) takes the place of reaction R5 over 600K to 3300 K and reaction R3 (C4F7N -> TS1 -> CF2CFCN -> CF4) becomes dominant above 700 K; reaction R15 (CF2CFCNCF3 -> CF2CFCN + CF3) plays the major role in the generation of CF3 with the overwhelming contribution rate. The results obtained here are expected to construct a relatively complete C4F7N decomposition scheme, including the main byproduct formation processes and to lay a theoretical basis for the investigation of its insulation performance.