Industrialization and urbanization have increased emissions of toxic atmospheric pollutants (CO, NO2, NH3), threatening ecological security and human health. Twin graphene (TG) features large specific surface area and excellent electronic transport properties, but its practical gas-capture performance is limited by weak physisorption. Silicon doping serves as an eco-friendly strategy to modulate TG’s electronic structure and generate surface active sites. Herein, first-principles calculations based on density-functional-theory were implemented. Six Si-modified TG models were built, among which the most stable ring-substituted TG-Si1 and short-chain central-doped TG-Si6 were further investigated. Their adsorption-sensing behaviors toward CO, NO2, NH3 as well as interfering H2O and O2 were systematically explored, and adsorption-desorption features at 298 K, 398 K and 498 K were evaluated. Results show that TG-Si1 produces strong chemisorption for NO2 and NH3 (Eads < −2.35 eV) suitable for long-term gas capture, though suffering from severe cross-interference. In comparison, TG-Si6 exhibits high selectivity for NO2; interfering gases can be readily desorbed at 498 K to realize good recyclability. Both Si-doped configurations possess favorable resistance against H2O and O2 interferences. This work provides theoretical support for Si-modified TG applied in toxic-gas adsorption-sensing and offers new insights into modification of two-dimensional carbon-based materials.