Antibiotics with lower metabolic levels such as tetracycline (TC) and ciprofloxacin (CIP) can cause serious pollution to water bodies, biochar (BC) is commonly used for the treament of wastewater polluted by these antibiotics. Activation is one of the main methods to improve the adsorption properties of biochar. With bagasse as the raw material, a novel modified biochar, namely KFBC, was prepared using KOH as the activator and FeCl3·6H2O as the impregnating solvent; in the meantime, bagasse BC was also prepared for the study as the control. Firstly, the effects of KOH activation and FeCl3 doping on the structural and physicochemical properties of KFBC were analyzed by various characterization methods. Then, the adsorption properties and mechanism of KFBC on tetracycline or ciprofloxacin in solutions were investigated by the adsorption method. The results show that more carbon structure defects form after KOH activation, which affects the type and quantity of functional groups and surface electricity of BC. The doping of FeCl3 enhances the magnetic properties of BC and facilitates the recycling of KFBC. In addition, pH has little effect on the adsorption reaction, which indicates that the adsorption by KFBC act in a wide pH range. The adsorption of TC and CIP by KFBC is featured by chemisorption, which is a spontaneous and endothermic process with increasing freedom and entropy. The theoretical maximum adsorption capacity of TC and CIP by KFBC can reach 743 and 794 mg/g. The dominant mechanism of KFBC is not electrostatic adsorption but pore diffusion and π—π interaction, hydrogen bond and complexation also play an important role.