In this work, the adsorption of four types of SF6 decomposed products over the perfect and defective (containing one S or W vacancy) WS2 monolayer was theoretically investigated using the density functional theory (DFT). By calculating the adsorption energy, electron transfer and comparing the band structure and the density of states (DOS) before and after gas adsorption, the physical and chemical interactions can be evaluated. The results show that the adsorptions of gas molecule over the perfect are mainly physisorption with low possibility of new bond formation. In the present of S vacancy, the SO2 adsorption can repair the S vacancy with an energy barrier of 1.28 eV and also, the adsorption leads to the significant increase of the band gap and high possibility of new bond formation. The introduction of W vacancy could not increase the adsorption energy for all gas molecules. However, it enhances the electron transfer between the SO2/H2S and the WS2 monolayer. The larger electron transfer may lead to the higher response of strong n-type or p-type WS2 monolayer toward the detected gas molecule. This work may provide prospective insight into the future application of the defective WS2 monolayer as gas sensing device.