Abstract: Biodegradable plastics have been evaluated as promising alternatives for conventional polymers in various fields. However, the adsorption behaviors and mechanisms of antibiotics onto biodegradable microplastics during their natural degradation remain poorly understood. To shed light on this issue, we conducted adsorption/desorption experiments on oxytetracycline (OTC) by polylactic acid (PLA) microplastics, comparing pre- and post-biodegradation conditions. The results showed that after 35 days of PLA natural degradation in wastewater, a dense biofilm formed on PLA surface, causing a decrease in the infrared intensity of ester functional group characteristic peak and suggesting bond rupture and degradation. The Langmuir isotherm model analysis revealed an initial adsorption capacity of 581.19 μg/g for OTC on PLA. The biofilm's presence increased the adsorption by 20.15%, primarily due to the complexation between OTC and biofilm's N—H functional groups. After biofilm removal, more oxygen-containing groups were exposed on the PLA's surface, enhancing hydrogen bonding and boosting OTC adsorption by 39.01%. The adsorption capacity of OTC on PLA increased first and then decreased with the increase of initial solution pH. Fulvic acid significantly influenced adsorption, reducing it by 13.25%-61.79% when presented at 200 mg/L. In summary, PLA's biodegradation enhances its OTC adsorption, potentially hindering antibiotic migration and transformation in water. This process highlights the ecological risk associated with microplastics carrying antibiotics, as they can accumulate in organisms. Revealing the OTC adsorption behavior of PLA during biodegradation contributes to a comprehensive understanding of degradable microplastics' environmental impact.