| To search for high sensitivity of graphene-based gas sensors for SO2 gas, the adsorption of SO2 molecules on pristine graphene (PG) and modified graphene, including single vacancy defect (SVG), Stone-Wales defect (SWG), Mn dopant (Mn-PG) and combining Mn dopant and defect (Mn-SVG and Mn-SWG), were investigated by first-principles based on density functional theory. It is found that SO2 molecule undergoes weak interaction with PG and SWG, indicating that PG and SWG are not sensitive to SO2 molecule; For SVG, SO2 adsorbing can effectively tune the change of electronic structure and SVG transforms from metallic to half-metallic. However, SO2 is still weakly adsorbed on SVG with relatively small adsorption energy of 0.636 eV. The substrate of Mn-SVG combines Mn dopant and single vacancy in graphene to enhance its interaction with the gas, but it demonstrates few changes in the electrical structure and magnetism by SO2 adsorption. Compared with other substrates, SO2 molecule can be adsorbed steadily on Mn-PG and Mn-SWG and SO2 adsorbing leads to remarkable reduction of magnetic moment and significant change in electrical conductivity for the system of Mn-PG and Mn-SWG. Therefore, Mn-PG and Mn-SWG can behave as ideal materials for detecting and removing SO2 gas in environment. Our work provides theoretical reference to design novel graphene-based gas sensor.