In this study, wheat straw was used as a raw material to prepare biochar (BC), and an in situ impregnation and carbonization method was adopted to prepare Fe-Mn-modified BC (FMBC) composites for the adsorption of antimonite (Sb(Ⅲ)) in water. Adsorption kinetics, response surface curves, and isothermal adsorption were utilized to conduct in-depth research on the adsorption performance of FMBC for removing antimony in water. Modern characterization techniques such as X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS) were used to investigate the mechanism of antimony adsorption from water by FMBC. Results showed that FMBC had stronger adsorption performance for Sb(Ⅲ) than wheat straw BC, with the maximum adsorption amount of 23.76 mg·g⁻¹ at 35°C. In the kinetic experiments, the quasi-secondary kinetic model could efficiently describe the whole adsorption process, which indicated that the adsorption process was mainly dominated by chemisorption. After the response surface optimization, the removal rate of Sb(Ⅲ) could reach 93.73% under optimal conditions. The results of isothermal adsorption experiments showed that the adsorption process was considerably in line with the Langmuir model, indicating that this adsorption was inclined to monomolecular-layer adsorption. XPS and other analyses demonstrated that FMBC mainly consisted of iron oxides and manganese oxides and had a large specific surface area, with an abundance of oxygencontaining functional groups. The mechanism of Sb(Ⅲ) adsorption by FMBC primarily comprised redox reactions, complexation, and ion exchange. The above results indicated that the Fe–Mn-modified wheat straw BC could be used as an effective adsorption material for the treatment of Sb(Ⅲ)-containing wastewater.