Potato stems and leaves biochar (PB) was prepared by pyrolysis at a temperature of 500°C under anoxic conditions. In order to strengthen the adsorption capacity, biochar was modified with alkaline solution (alkali modified biochar, APB). Two kinds of biochars were adopted as adsorbents to remove ciprofloxacin (CIP) from aqueous solution. The adsorption behavior of CIP onto biochar before and after alkali modified including adsorption kinetics and isotherms were investigated. The effects of different factors (equilibrium time, pH, temperature and initial concentration) during the adsorption process were also investigated. Biochar samples were characterized by Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and nitrogen adsorption–desorption isotherm. The results showed that the alkali treated biochar possessed more mesopores than raw biochar, and accordingly exhibited a more excellent adsorption performance (23.36 mg·g−1) than raw biochar. Hydrophobic interaction, hydrogen-bonding interaction, electrostatic interaction, and π − π interaction were the adsorption mechanisms for CIP uptake onto the two adsorbents.