<p>In addressing the issue of cadmium (Cd²⁺) contamination, a novel iron and phosphorus co-doped biochar modified geopolymer composite (Fe–P-BC/GM) was prepared utilizing corn straw and kaolin as precursors through an impregnation-calcination method, and was employed to adsorb Cd²⁺ in water and soil. Under optimal conditions (adsorbent dosage: 10 mg, pH: 7), the maximum adsorption amount of Cd²⁺ by Fe–P-BC/GM in aqueous solutions was 286.6 ± 13.2 mg·g⁻¹. In soil remediation experiments, applying 1 <i>wt</i>% Fe–P-BC/GM promoted the transformation of cadmium from acetic acid-extractable fraction (F1) with high bioavailability to reducible (F2), oxidizable (F3), and residual (F4) fractions with lower bioavailability. Pot experiments, along with analyses of plant enrichment and translocation factors, confirmed that Fe–P-BC/GM suppressed the translocation of Cd²⁺ from soil to plants. Adsorption experiments and XPS analysis further elucidated that the adsorption of Cd²⁺ was governed by synergistic mechanisms, primarily surface complexation and chemical precipitation, with ion exchange playing an important role. Additionally, electrostatic interactions, coupled with redox mediation, collaboratively enhance the process. This research offers a novel approach for addressing Cd²⁺ pollution in the environment and promotes the high-value utilization of industrial and agricultural waste materials.</p>