In this study, we innovatively synthesized Ru-CeO2 catalysts by doping ruthenium dioxide (RuO2) with cerium dioxide (CeO2), which significantly enhanced the removal efficiency of mercury (Hg0) from coal-fired flue gases and tolerance to high sulfur environments. We effectively responded to the complex challenges in coal-fired flue gases. Nitrogen adsorption-desorption, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) were employed to characterize the catalyst. It was found that the mercury removal efficiency of the Ru-CeO2 catalyst was significantly higher than that of unmodified CeO2 at 125°C, demonstrating excellent mercury removal performance and sulfur resistance. In addition, in the field of mercury removal and sulfur resistance, the innovative combination of characterization technique and density functional theory (DFT) was used to deeply explore the adsorption behaviors of Hg0 and SO2 on the surface of the catalyst and their mechanisms, revealing the changes in surface charge distribution and the increase in active sites caused by ruthenium doping. The changes in the electronic structure of the active sites on the catalyst surface were revealed by DFT calculations, which provide a scientific basis for the optimization and screening of catalysts in the future and are of great significance to the field of coal-fired flue gas purification and catalyst science.