In this work, we reveal the influencing factors and mechanisms of tetracycline removal by peroxydisulfate activated with boron-modified sewage sludge biochar in urine. Advanced characterization techniques showed that the introduced BCO2 and BC2O were the main activation sites for peroxydisulfate, which maintained a high removal rate of tetracycline in synthetic and actual urine (86.48% and 77.35%). HCO3−/CO32− and some organic matter in the urine contributed to the degradation of the tetracycline. Theoretical calculations showed that CO32− in urine could be efficiently adsorbed by the introduced B atoms and converted to CO3·− by peroxydisulfate to assist the degradation of tetracycline, which was attributed to the special electronic structure of the B atoms as well as the strong electron-enrichment ability of the neighboring O atoms. The potential toxicity of tetracycline and its degradation intermediates was further explored, revealing a significant reduction in toxicity during boron-modified sewage sludge biochar/ peroxydisulfate treatment.