Abstract: Antibiotics are widely used in the livestock and poultry breeding industry to enhance disease resistance and growth rate. However, the residual antibiotics can enter the water and soil environment through manure, easily leading to an increase in bacterial resistance in the environment and thus posing a potential threat to the ecological environment. Traditional fecal treatment methods, such as aerobic composting and anaerobic fermentation, have long degradation cycles and limited effects on antibiotics. In contrast, hydrothermal treatment, as an efficient thermochemical conversion method, has been proven to effectively promote the rapid and complete degradation of antibiotics. In this study, oxytetracycline, which has the highest proportion of antibiotic residues in pig manure, was selected as the research object. The oxytetracycline - water model system was established and optimized using Materials Studio software. Combined with LAMMPS software and the ReaxxFF reaction force field, the molecular dynamics simulation of the degradation process of oxytetracycline under hydrothermal conditions was carried out. The evolution law of the hydrothermal degradation of oxytetracycline in the hydrothermal system and the hydrothermal reaction path were explored. The changing trends of water and hydroxyl radicals in the system and their mechanisms of action in the reaction were analyzed. Results indicate that hydrothermal temperature is the critical factor controlling oxytetracycline degradation. As temperature increased, long-chain oxytetracycline molecules progressively fragmented into short-chain small-molecule fragments: the proportion of C_1～5 fragments rose from 45.45% at 1 800 K to 80.68% at 3 000 K, while the proportion of complex long-chain molecules with C_>15 decreased from 51.52% to 6.82%. The hydrothermal degradation of oxytetracycline proceeded through two stages characterized by rapid and slow increases in fragment formation, and molecular decomposition intensified with rising simulation temperature. In the simulated temperature range of 1 800 K to 2 000 K, the total number of molecular fragments produced by the degradation of oxytetracycline in the system increased slowly. However, when the temperature further rose to 2 200 K, the total number of molecular fragments produced by the degradation of oxytetracycline in the system increased significantly. The simulations also showed that changes in the number of H₂O molecules in the system were closely related to the dominant reaction types. At the initial stage of the reaction at a lower hydrothermal temperature (1 800～2 200 K), the number of H₂O molecules in the system is lower than the initial value, and oxytetracycline mainly undergoes hydrolysis reaction in the system. However, as the hydrothermal temperature rises, the number of H₂O molecules in the system gradually increases, and dehydration reaction mainly occurs in the system. In addition, the research also found that as the hydrothermal temperature increased, the number of hydroxyl radicals in the system gradually increased and fluctuated within a higher number range. These hydroxyl radicals preferentially attack high-electron-density active sites on the oxytetracycline molecule, promoting a series of complex reactions such as demethylation, deamination, dehydration, hydroxylation, and ring-opening of oxytetracycline. Ultimately, it drives the degradation of oxytetracycline into small molecule substances. By elucidating the hydrothermal degradation mechanism of oxytetracycline at the molecular level through reactive molecular dynamics simulations, this study provides theoretical guidance and a novel perspective for the efficient removal of antibiotics from swine manure.