Organic pollutants in water are an increasingly prominent problem. Given this challenge, this study investigated the high adsorption capacity of reed-based biochar for use as an adsorbent using the potassium hydroxide (KOH) activation method. We investigated the performance and mechanism of reed-based biochar with respect to the adsorption of a significant contaminant of emerging concern, tetracycline (TC). The effects of pH, contact time, temperature, and initial pollution concentration on the adsorption rate were investigated in detail. The experimental results suggest that the internal structure of activated biochar was loose and porous, and the specific surface area (BET) increased by 194.08 times, reaching 965.31 m² g⁻¹ after KOH activation. The biochar surface was electronegative, due to the ionization of oxygen-containing functional groups, such as hydroxyl (–OH) and carboxyl (–COOH) groups. Solution pH had only a weak influence on TC adsorption; neutral conditions favored adsorption. The adsorption kinetics and isotherms were represented well by the pseudo-second-order and Freundlich models, respectively. The chemical multilayer adsorption may play an important role in TC adsorption, which was a spontaneous endothermic reaction. The adsorption process occurred more easily after KOH activation and the adsorption capacity of biochar improved by more than 20 times. These results indicate that preparing reed-derived biochar using KOH activation is an effective way to reduce pollution and utilize a waste resource.