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H2S is a kind of impurity in the alternative energy resource of biogas and landfill gas. Among the removal methods, absorption by alkanolamines is the commonly accepted and widely used ones. This is due to the efficient regeneration. In this study, the absorption mechanism of H2S by two alkanolamines, namely diethanolamine (DEA) and N-methyldiethanolamine (MDEA), was proposed in a different way and analyzed by DFT calculations. The structural, energetic, red shifts and topological analysis of the most stable structures were then investigated. The values of electron densities and Laplacian densities at the bond critical points fall in the range of a hydrogen bond. Two kinds of hydrogen bonds S–H···N and O–H···S formed between H2S and DEA/MDEA are considered to be the driving forces during the absorption process. Thus, it can be known that H2S can act as both a hydrogen bond acceptor and a donor. Besides, the S–H···N hydrogen bond is much stronger than the O–H···S hydrogen bond. According to the change rules of Gibbs energies, the formation of hydrogen bond becomes difficult with increasing temperature and deceasing pressure. The absorption mechanism of H2S captured by alkanolamines via hydrogen bond interaction provides a novel theoretical direction for further study. [ABSTRACT FROM AUTHOR]