Antibiotic contaminants such as tetracycline (TC) from agricultural production have become widely distributed and persistently accumulated in aquatic environments (rivers, lakes, and oceans), posing severe threats to ecological security and human health. This study developed a modified rice-straw-derived biochar through NaOH activation and ball-milling-assisted Fe₃O₄ loading, which simultaneously enhanced TC adsorption capacity and enabled magnetic recovery. The Box–Behnken design (BBD) response surface methodology was employed to optimize three key preparation parameters: ball-milling time (A, 39.95 min), frequency (B, 57.23 Hz), and Fe₃O₄/biochar mass ratio (C, 2.85:1), with TC adsorption capacity as the response value. The modified biochar was systematically characterized using SEM, BET, FTIR, XRD, and XPS, while adsorption mechanisms were elucidated through kinetic studies, isotherm analyses, and pH-dependent experiments. The results demonstrate that modification via ball-milling with Fe₃O₄ loading significantly enhanced the biochar’s tetracycline adsorption capacity. The maximum adsorption capacity of the modified biochar reached 102.875 mg/g, representing a 114.85% increase from the initial value of 47.882 mg/g observed for the pristine biochar. Furthermore, the modified biochar exhibited excellent stability, maintaining robust adsorption performance across a wide pH range. The primary adsorption mechanisms involved metal coordination complexation, supplemented by hydrogen bonding, π-π interactions, and pore filling.