Abstract:   Tetracycline (TC) is frequently used in fishery farming as an antibacterial agent. However, when it enters the wastewater and is discharged immediately without sufficient treatment, a low usage rate will cause pollution of the nearby water environment. This work employed high-temperature calcination derivatization to create nano-magnetic triiron tetroxide composites with carbon and nitrogen coatings (Fe₃O₄/CN) on an iron alginate hydrogel precursor substrate. Fe₃O₄/CN was structurally characterized using XRD, FT-IR, and TEM. Its effectiveness and method of photocatalytic degradation of TC in water under simulated solar light were also investigated. The results showed that the removal rate of TC (50 μmol/L) could reach more than 95.0% when the water's pH was 7, 0.05 g/L of Fe₃O₄/CN was added, and the photocatalytic reaction lasted 150 minutes. Results from Raman, EIS, and ESR show that Fe₃O₄/CN has a high charge separation efficiency and a large number of defects; Under solar radiation, TC is directly oxidized, primarily through a non-radical pathway that produces holes (h⁺), reactive material can be recovered by magnet separation. HPLC-MS discovered the photocatalytic reaction's intermediates, TC primarily underwent oxidation reactions such as deamidation and ring opening under the influence of h⁺. QSAR model prediction showed that the toxicity of the intermediates was decreased. Therefore, removing TC from agricultural wastewater using photocatalytic technology based on Fe₃O₄/CN has some potential application value.