Abstract: Selective catalytic reduction (SCR) technology is a key technology for industrial flue gas denitrification. However, traditional SCR catalysts suffer from low efficiency below 200°C. The Mn-based catalysts exhibit high catalytic performance and great application potential under low temperature, but systematic studies on monolithic Mn-based catalysts are lacking. In this article, Sm-Mn/Ti-y (y = 3, 1.5, 1) monolithic honeycomb catalysts with varying active component loads are prepared by co-precipitation combined with mixing-extrusion molding method. Honeycomb catalysts with no cracks on the surface, good smoothness, and excellent molding ability are obtained. The performance test results demonstrate that the Sm-Mn/Ti-1.5 catalyst exhibited over 90% NO conversion and 70% N2 selectivity between 100-180°C. Characterization and testing techniques, including XRD, FESEM, N2 adsorption-desorption tests, XPS, H2-TPR, and NH3-TPD, are conducted to explore the effect of the Sm-Mn/Ti ratio on the surface phase, structure, species, redox capacity, and adsorption capacity of the catalyst. The XRD results indicate that only TiO2 phase exists, with no other phases detected, this indicates that the Sm and Mn elements are uniformly dispersed on the catalyst surface and do not form any long-range ordered lattice. The SEM characterization results show that the catalysts are consisted of colonies and nanoparticles on their surfaces. Highly dispersed element distribution and small surface structure are beneficial for improving the catalytic performance. N2 adsorption-desorption tests indicate that the catalysts are consisted of the mesoporous structures. The XPS characterization, H2-TPR and NH3-TPD test results reveal that the Sm-Mn/Ti-1.5 catalyst possesses the highest Mn4+ content (56.5%) and weak acid site amount (280.7 μmol·g-1) among the tested samples. The elevated Mn4+ concentration enhances the catalytic activity by facilitating the activation of more reactive species. While the increased amounts of weak acidity sites provide sufficient adsorption sites to promote the conversion of NO and reduce the formation of N2O. Notably, the XPS characterization results also show that the Sm-Mn/Ti-1.5 catalyst exhibits a lower ratio of Sm3+/Sm (24.10%) than Sm-Mn/Ti-3, and a lower ratio of Oα/O (25.12%) than Sm-Mn/Ti-1. This effectively regulates the double redox cycle of Mn4+/Mn3+ and Sm3+/Sm2+, sustaining high activity while mitigating N2 selectivity. It is found that Sm-Mn/Ti-3 has a high NO conversion but a low N2 selectivity due to excessive Sm3+/Sm and decreased acidity on its surface. Although Sm-Mn/Ti-1 has the highest ratio of Oα/O, its low NO conversion is due to the fewer amount of Mn4+species and weak acid sites. In summary, the synergistic optimization of the oxidation ability and acidic sites of Sm-Mn/Ti catalyst is achieved by regulating the loading of active components, resulting in a Sm-Mn/Ti-1.5 catalyst that balances high NO conversion rate and good N2 selectivity. In order to verify the SO2 poisoning resistance of the Sm-Mn/Ti-1.5 catalyst, the NH3-SCR experiments are carried out in a 50 ppm SO2 atmosphere at 120°C. The results show that the high NO conversation rate and N2 selectivity maintained for 12 h without significant decrease, which demonstrates that the Sm-Mn/Ti-1.5 catalyst also exhibits excellent SO2 resistance. The present study achieves the formation and optimization of active components of Mn-based catalysts, offering a new strategy for efficient low-temperature SCR catalyst development.