Arsenic (As) pollution poses a serious threat to both ecological systems and human health, making the development of economical and efficient remediation materials a key research priority. Steel slag (SS) and corn straw biochar (BC) are promising low-cost precursors with strong potential for As immobilization. In this study, steel slag-corn straw biochar composites (SSBCs) were produced via co-pyrolysis to elucidate their mechanisms for immobilizing As in contaminated paddy soils and inhibiting As uptake by ryegrass. Through ryegrass pot experiments and adsorption analyses, the As-reduction performance of BC, SS, and SSBCs was systematically evaluated, with particular emphasis on their effectiveness in lowering As bioavailability in paddy soil. Pot experiments revealed the application of 2 % SSBCs increased ryegrass biomass while markedly decreasing As accumulation in ryegrass roots. Furthermore, SSBC, (SS: BC= 1:1, mass ratio) and SSBC2 (2.5:1) treatments elevated soil pH and significantly reduced available As concentrations. Characterization analyses indicated that the enhanced As(III) removal efficiency of SSBC, was primarily attributable to its hydroxyl, Ca-O, and Fe-O functional groups and its porous structure, which promoted surface complexation and electrostatic adsorption. In adsorption experiments, both BC and SSBCs rapidly adsorbed As(III), reaching equilibrium within 120 min. Among them, SSBC, exhibited the highest adsorption capacity (21.64 mg g-1), representing a six-fold increase compared with BC (3.46 mg g-1). Adsorption capacity increased under acidic conditions (pH 3.0-7.0), and SSBC, significantly enhancing soil As(III) retention. These mechanisms contributed to effective As immobilization, thereby reducing its bioavailability and subsequent plant uptake. Overall, the preparation of SSBCs improved the physicochemical properties and surface structure of BC, enhanced its adsorption performance, and demonstrated strong potential for the remediation of As-contaminated paddy soils.