Recently, the application of phosphorene structure analogues in gas sensors has been a hot research topic since the appearance of phosphorene. SnSe monolayer as one of them has been proved to be much more stable properties than phosphorene. Based on the density-functional theory, the interaction between gas molecules (CO, CO2, O-2, NO, NH3, SO2 and NO2) and SnSe monolayer are theoretically investigated by first-principles calculation. Macroscopically, gas molecules selective adsorption of SnSe monolayer is analyzed by molecular dynamics. Compared to CO, CO2, O-2, SnSe monolayer performs stronger affinity for SO2 and NO2, which possesses appropriate adsorption energies (-6.000 eV and -0.759 eV) and elevated charge transfers (-0.239 e and -0.328 e). SnSe monolayer chemical adsorption of NO2, while physically adsorbing SO2, is more suitable for the adsorption mode of SO2 sensors. Surprisingly, the adsorption amount of SO2 is 6 times that of NO2. Therefore, the adsorption of SO2 is more likely to occur compared to other gas molecules. For a mixed environment of SO2 and NO2, the adsorption quantity of SO2 is not significantly affected, while the adsorption of NO2 is inhibited. Therefore, the SnSe monolayer could be a promising candidate as SO2 sensors with high selectivity and sensitivity.