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This study investigated the adsorption performance of antibiotics using modified semi-carbonized fibers (SCF) prepared from polyacrylonitrile and loaded with metal ions. Nitrated semi-carbonized fibers (N-SCF) and magnetized–nitrated semi-carbonized fibers (MN-SCF) were prepared through nitrification and magnetization methods, and different metal ion-loaded MN-SCFs were obtained by loading Ca and Mg ions onto MN-SCF. The effects of pH, temperature, and ionic strength on antibiotic adsorption were analyzed. The adsorption kinetics and thermodynamics of tetracycline (TC), chlortetracycline (CTC), and oxytetracycline (OTC) were studied using batch experiments. Results showed that (1) the adsorption of antibiotics on the tested materials initially increased and then decreased with increasing pH and ionic strength, reaching maximum values at pH 4 and 0.1 mol/L, respectively. (2) Higher temperatures facilitated the adsorption of antibiotics on the materials, which followed a chemisorption mechanism. (3) The Langmuir model provided a better fit for the adsorption isotherms of TC, CTC, and OTC compared with the Freundlich model, suggesting monolayer adsorption. (4) The maximum adsorption capacity (qm) of the antibiotics ranged from 0.80 to 3.97 mg/g, with of CTC > OTC > TC. Metal ion-loaded MN-SCFs exhibited higher adsorption amounts compared with MN-SCF alone.