The carboxyl-modified biochar derived from palm fiber was prepared through pyrolysis and oxidation. The adsorption behavior for Pb(II), Cu(II) and Cd(II) on the carboxyl-modified biochar was systematically investigated. The experimental results demonstrated that the carboxyl-modified biochar exhibited impressive adsorption performance owing to its multi-porous structure and abundant adsorption sites on the surface. The maximum adsorption capacities for Pb(II), Cu(II) and Cd(II) reached 218.5, 132.1 and 79.9 mg/g, respectively. The adsorption kinetics and isotherm could be well described by the pseudo-second-order model and the Langmuir isotherm model, respectively. After five successive adsorption/desorption cycles, the removal efficiency was still more than 90%, 86% and 83% for Pb(II), Cu(II) and Cd(II), respectively.