For exploring the gas sensitivity of SnSe2 single atom 2D layer, geometry optimization and energy calculation were carried out to search out the best adsorptive site and sorption capacity for SnSe2 single layer adsorbed respectively with H2, CO, NH3 and NO2 gas molecules. The parameters are calculated based on the first principle method such as Density Functional Theory(DFT) including bond length and its angle, band structure, total density of state, electronic density and charge difference density. The relationship between the electronic structure changes and the gas sensing was studied. The result reveals that H2 and CO cannot change the electronic structure of SnSe2 single layer, but fresh impurity level is generated because of adsorption of NO2 and NH3. This new level locates between Conduction Band Minimum(CBM) and Valence Band Maximum(VBM) and pushes the Fermi level down to VBM by NH3, or up to CBM by NO2 in shallow level. The charge difference density show that nothing changed by H2 and CO adsorbed on SnSe2 single layer, but some electrons gathering occurred locally on sorption site in the layer adsorbed by NO2, and electrons shifted to surface atom after adsorbing NH3. It is distinct that SnSe2 2D single layer has the obvious gas sensitivity for NO2 and NH3 and the good selectivity for NO2 as well.