Purpose <p>This study aimed to investigate the effects of the Bt toxin concentration on the adsorption characteristics of cadmium ions in red and brown soils and to further elucidate the mechanism of action.</p> Methods <p>Batch experiments were conducted to investigate the kinetic and equilibrium characteristics; fluorescence quenching tests were carried out to investigate the number of binding sites of Cd(II) with Bt toxin; and SEM, FTIR, and XPS were used to analyze the mechanism of action.</p> Results <p>The results revealed that Bt toxin (0–10&#xa0;mg/L) enhanced Cd(II) adsorption in red and brown soils. In the absence of Bt toxin, the maximum Cd(II) adsorption capacities (<i>q</i>_<i>max</i>) in red and brown soils were 0.1174 and 1.724&#xa0;mg/g, respectively; when the concentration of Bt toxin was 10&#xa0;mg/L, the <i>q</i>_<i>max</i> values for Cd(II) adsorption in red and brown soils increased to 0.1398 and 1.794&#xa0;mg/g, respectively. The adsorption kinetic characteristics of Cd(II) in both types of soils followed the pseudo-second-order kinetic model, suggesting that chemisorption also occurred during adsorption. XPS and FTIR analyses supported the Cd(II) chemical adsorption mechanism. According to a fluorescence quenching study, the Bt toxin and Cd(II) binding ratio was 1:2. The complexation of two cadmium ions by one Bt toxin molecule may be the mechanism by which Bt toxin promotes Cd(II) adsorption.</p> Conclusion <p>The adsorption of cadmium ions in the soil is increased by Bt toxin, indicating that the risk of cadmium pollution may change. The present work sheds new light on the impact of Bt toxin on the fate and risk of Cd(II) in agricultural soil. The migration, transformation and biological availability of cadmium ions in the soil deserve further study.</p>