<p>Selective catalytic reduction (SCR) technology is widely applied in the denitrification process of coal-fired power plants. It is known that catalyst plays an important role in SCR technology. However, due to the instability of working conditions and complex flue gas environment, the lifespan of the catalyst is limited, resulting in the generation of large amounts of spent SCR catalysts in industry. Studying the deactivation mechanisms of spent SCR catalyst is the key theoretical basis for achieving their efficient regeneration and resource recovery. However, the understanding of deactivation mechanisms for spent SCR catalyst remains limited. In this study, a comprehensive analysis was conducted using characterization techniques such as X-Ray Fluorescence (XRF), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), and Fourier Transform Infrared Spectroscopy (FT-IR) measurements. The physicochemical properties of fresh and spent SCR catalysts were examined. Results indicated local agglomeration occurred on the surface of spent catalyst. The main components of spent catalyst were anatase TiO₂. The valence states and chemical structures of main components remained unchanged. Compared with the fresh catalyst, the spent catalyst showed a decrease in the content of main components, the number of acidic sites, and the specific surface area. These findings collectively summarized the key analytical outcomes of the study. Based on these findings, the deactivation was attributed primarily to two factors: Firstly, the poisoning of active sites by alkali metals (K, Na) and arsenic (As) from the flue gas, which sharply reduced active site availability and weakened NH₃ adsorption. Secondly, the erosion by high-temperature and high-velocity flue gas led to the deposition of fly ash and sulfate, resulting in pore channel blockage and the loss of some active components. This conclusion integrated the characterization results to explain the deactivation mechanisms.</p> Graphical Abstract <p></p>