In this paper, based on density functional theory, use the most stable modified Ag, Au, and Co atoms to modify the SnS2 monolayer, and the adsorption mechanism and gas sensitivity of NH3, Cl2, and C2H2 gas are analyzed. The results are analyzed using an optimal mechanism, adsorption parameters, electron deformation density, density of states, frontier molecular orbital theory, and desorption performance. The results show that all the adsorption of gas molecules is exothermic and spontaneous, and chemically adsorb NH3, Cl2, and C2H2 gas in the same method. The adsorption energy of the Ag-SnS2 monolayer to C2H2 gas is − 0.871 eV, and the recovery time after heating is shortened rapidly, which is very suitable for detecting C2H2 gas. The adsorption energy of Au-SnS2 monolayer for Cl2 gas is − 1.878 eV, which is much larger than other adsorption conditions and can be used as a dedicated Cl2 gas adsorbent. The Co-SnS2 monolayer is also not easy to desorb these three gases after heating and can be used as a general adsorbent for NH3, Cl2, and C2H2 gases.