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Although layered MoS _2 has been proposed as a potential candidate for gas detection devices due to high surface-to-volume ratio, high sensitivity, and selectivity, the adsorption behavior of small molecules is still ambiguous. Here, we performed the first-principles calculations to investigate the adsorption behavior of small molecules on layered MoS _2 surface, and the effects of defects and environment are considered. Our results reveal that NO and NO _2 can be chemically adsorbed on defective monolayer MoS _2 , which is attributed to the forming of covalent bonds. And the forming of covalent bonds can lead to an increase in adsorption energies. Whereas, gas molecules can only be physically adsorbed on perfect MoS _2 . Meanwhile, as compared with adsorption behavior of NH _3 , NO, and NO _2 on clean MoS _2 , the environmental gases (CO _2 , N _2, and H _2 O) may result in an increase in adsorption strength of NH _3 , NO, and NO _2 on MoS _2 surface.