In this study, biochar was prepared using sesame straw and treated with slightly soluble phosphate. The removal effect, adsorption mechanism, and influencing factors of biochar on Pb(II) in water were determined using batch adsorption experiments. Chlorapatite-modified biochar (BC-Cl) and hydroxyapatite-modified biochar (HBC) were prepared by co-impregnating the original biochar (BC) with hydroxyapatite and BC. The results showed that BC-Cl exhibited the best removal effect on heavy metal Pb(II) at pH = 5 and a dosage of 600 mg·L−1. The adsorption of Pb(II) by BC-Cl was in line with the pseudo-second-order kinetic equation and the Langmuir isothermal adsorption model, indicating that the adsorption process was dominated by chemical adsorption and monolayer adsorption. Additionally, phosphorus was essential in the adsorption of Pb(II) by the modified biochar. The main reason for the high removal rate of Pb(II) by BC-Cl is that PO4 3- produced via chloroapatite dissolution can precipitate with Pb(II) and then generate insoluble phosphate precipitation in water. In summary, the inorganic material apatite-modified sesame straw biochar has excellent application potential as it has a high removal rate and excellent adsorption performance for Pb(II) in water.