In this study, iron and nitrogen co-doped biochar (Fe-N-BC) was synthesized by a facile pyrolysis process of mixed walnut shell, melamine, and iron(III) chloride, and then the biochar (BC), nitrogen-doped biochar (N-BC), and iron-doped biochar (Fe-BC) also were prepared as a control. The Fe-N-BC demonstrated an excellent degradation performance (0.5 min-1) for sulfamethoxazole (SMX) in the presence of peroxymonosulfate (PMS) compared with BC (0.026 min-1), N-BC (0.038 min-1), and Fe-BC (0.33 min-1). The transformation products of SMX during the degradation process were also identified through ultra-high performance liquid chromatography in combination with high-resolution mass spectrometry (UHPLC-QTOF-MS), and the predicted toxicity results from Toxicity Estimation Software Tool (T.E.S.T) exhibited that the overall toxicity of the degradation products lower than SMX.