The adsorption characteristics of Pb²⁺ in water by rice-straw biochar were elucidated by adsorption isotherm and kinetics, and the adsorption mechanism of biochar was qualitatively and quantitatively revealed by means of elemental analysis, BET (Brunauer-Emmet-Teller) -N₂ specific surface area analysis, Zeta potential, scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), along with the calculation for relative distribution of each adsorption mechanism. The results indicated that the maximum adsorption capacity of Pb²⁺ by biochar were 233.48 mg/g, 236.00 mg/g and 237.50 mg/g under the temperatures of 20 ℃、30 ℃ and 40 ℃, respectively. The experimental data fitted the Langmuir (R² > 0.96) better than the Freundlich isotherm, which suggested monolayer adsorption and a homogeneous distribution of active surface sites. Under the initial concentrations of 50 mg/L, 100 mg/L and 300 mg/L, the equilibrium time of biochar were 30 min, 90 min and 360 min, respectively, which followed the pseudo-second-order mechanism and corresponded to a chemisorption process. The main mechanisms responsible for Pb²⁺ adsorption by rice-straw biochar included chemical precipitation, cation exchange, Cπ-electron coordination, and functional group complexation, in which the chemical precipitates could be mainly Pb₄ (CO₃)₂ (SO₄) (OH)₂ and PbCO₃, accounting for 47.15% ~ 50.81% of total adsorption. Cation exchange also played an important role in the adsorption mechanisms due to high contribution proportions to total adsorption (32.82% ~ 37.77%). Among the adsorption mechanisms, both the Cπ-electron coordination and functional group complexation could contribute 15.03% to 16.37% to the total adsorption, but these were overshadowed by the chemical precipitation and cation exchange, accounting for 83.63% to 84.92% in the total adsorption. These suggested rice-straw biochar could be an efficient adsorbent for the removal of Pb²⁺ from aqueous solution.