Activated carbon (AC) has the advantage of multi-pollutant control with a high removal efficiency for flue gas purification. It is necessary to determine the oxidation mechanism and oxidation products of hydrogen sulfide (H2S) on AC to simultaneously capture H2S and SO2. A fixed-bed reactor was coupled with a mass spectrometer (MS) to detect the gaseous components and X-ray photoelectron spectroscopy (XPS) was used to distinguish the sulfur-containing species in the oxidation products on AC and thermally treated AC to investigate the actions of the oxygen-containing functional groups. The results revealed that H2S was adsorbed on the AC surface and combined with oxygen-containing functional groups to form sulfate (SO42-) in the absence of O-2. In the presence of O-2, H2S and O-2 in the atmosphere were adsorbed on the AC active sites, and the catalytic reaction produced elemental sulfur. Elemental sulfur further reacted with O-2 to produce gaseous SO2. According to the detection results of XPS and the in situ diffuse reflectance infrared Fourier transform (in situ DRIFT) spectra, an oxidation mechanism of H2S by O-2 and oxygen-containing functional groups was described. The C=C and C = O bonds were broken by H2S, and then, C-S and S-O bonds of the intermediate products were generated. The C-S bond tended to form elemental sulfur, while the S-O bond likely formed a sulfate or gaseous SO2. Porosity analysis showed that the AC specific surface area decreased after H2S adsorption and that sulfate reduced the specific surface area much more than elemental sulfur.