In order to enhance the specific surface area of raw sludge-derived biochar and its adsorption ccapacity for ofloxacin (OFL) in aqueous solutions, a series of modified sludge-derived biochars (KBC) were prepared from municipal sludge as the carbon matrix through KCl-assisted pyrolysis combined with acid washing, and characterized by SEM, BET, FTIR, and XPS for analyses on their microstructural and physicochemical properties. The effects of acid washing method, adsorbent dosage, initial pH, temperature, and coexisting anions on OFL adsorption were evaluated, with the underlying mechanisms of the adsorption process elucidated. The results showed that biochar modified with 1 mol/L HCl (1M-KBC) exhibited the most effective mineral removal and pore structure development, yielding a high specific surface area of 163 m2/g and abundant oxygen-containing functional groups, which contributed to its superior OFL adsorption performance. Under optimized conditions of 30°C, neutral pH, and an adsorbent dosage of 0.6 g/L, the adsorption capacity of 1M-KBC for OFL reached 26.96 mg/g. The adsorption process of 1M-KBC for OFL was a spontaneous and endothermic process, which conformed to the Freundlich isotherm model and the pseudo-second-order kinetic model. Hydrogen bonding, π–π interactions, electrostatic interactions and pore filling were involved in the adsorption of OFL by 1M-KBC. 1M-KBC exhibited good regeneration ability, with the OFL removal rate decreasing by approximately 15% after five consecutive adsorption-desorption cycles.