Abstract: Biochar was prepared by pyrolysis using sludge from secondary sedimentation tank of sewage treatment plant as raw material. The physicochemical properties of biochar were characterized by scanning electron microscope, specific surface area and pore size analyzer, X-ray diffractometer and X-ray photoelectron spectroscopy. A sludge biochar/persulfate system was constructed and used for the degradation of sulfadiazine. The effects of the dosage, temperature, pH value and other reaction conditions on the removal of sulfadiazine were explored, and the removal mechanism of sulfadiazine in the system was analyzed. The results show that the pyrolysis process caused certain changes in the pore structure and surface composition of the sludge. The sludge biochar prepared at high temperature (700 ℃) performed better persulfate activation efficiency and the increase of temperature favored the degradation of sulfadiazine. In the range of pH value (3.8-8.8), chloride ion concentration (0-10 mmol·L^-1) and sulfate ion concentration (0-10 mmol·L^-1), the system could maintain high removal efficiency of sulfadiazine. High concentrations (10 mmol·L^-1) of carbonate, bicarbonate and hydrogen phosphate ions had certain inhibition function on the system. High concentration (10-100 mg·L^-1) of humic acid could obviously inhibit the degradation of sulfadiazine, but the actual water environment showed little effect on the system. The active species quenching test and electron paramagnetic resonance (EPR) detection proves that free radicals could play a major role in the degradation of sulfadiazine in the system.