The overuse of herbicide may cause pollution in water and soil environment, affecting the growth of crops. And how to reduce herbicide residues has aroused extensive attention. In this work, a novel modified biochar (Zn/SN-BC) was prepared with reed straw as raw material, ZnCl2 and thiourea as modifiers to remove quinclorac (QC) residues in water and soil. Compared with primitive biochar, Zn/SN-BC, benefiting from higher specific surface area (766.5 m2·g−1), improved pore characteristics and abundant reactive sites (−C = O, −COOR, −C-N, −C-S, ZnS, etc), showed excellent adsorption capacity towards QC (235.9 mg·g−1), which was much higher than others reported before. According to the characterization results, it was revealed that Zn/SN-BC adsorbed QC through pore filling, hydrogen bonding, π-π conjunction, electrostatic attractions and other chemical interactions. The column leaching and pot experiments demonstrated that Zn/SN-BC could reduce QC leaching and bioavailability in soil, and effectively alleviate its phytotoxicity on tobacco plants. This study gives a new insight into the application of biochar-based materials for the treatment of herbicide residues in agricultural environments.