The mechanisms of three cascade reactions between N-methylindole and sulfonylazides via Huisgen cycloaddition were investigated using density functional theory, and the polarized continuum model (PCM) was applied to simulate the solvent effects. Fukui functions and dual descriptor have been employed to assess the atomic reactivity. With respect to these three reactions, each has two possible reaction pathways (I and II). The calculated results show that singlet O-2 plays a crucial role in the oxidative dehydrogenation process in the first reaction between 2-imine indole and p-tosylazide (TsN3), when the reaction occurs without O-2 but with N-2, two pathways via the processes of breaking two covalent bonds, followed by 1,2-H shift and N-2 removal (or by N-2 removal and 1,2-H shift) exist. Water molecule plays an important role in the H-shift process in the third reaction of the methyl substituted N-methylindole with TsN3. Our results demonstrated that these reactions can take place at certain condition, in good agreement with the experimental observation. The understanding of the competitive pathways for Huisgen cycloaddition of N-methylindole and sulfonylazides can provide valuable insights into related reactions. (C) 2016 Elsevier B.V. All rights reserved.