Biodegradable mulch film is considered a promising alternative to traditional plastic mulch film. However, biodegradable mulch film-derived microplastics (BMPs) in the environment have been reported as carriers for herbicides. Particularly in agricultural settings, limited attention has been given to the abiotic and biological aging processes of BMPs, as well as the herbicides adsorption mechanisms and associated health risks of BMPs. This study investigated the adsorption behaviors and mechanisms of mesotrione on both virgin and aged polylactic acid (PLA)/poly (butylene adipate-co-terephthalate) (PBAT) BMPs, and further evaluated their bioaccessibilities in gastrointestinal fluids. A variety of physical and chemical methods, including scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS), revealed increased roughness, generation of oxygen-containing functional groups, and higher O/C ratios of PLA/ PBAT BMPs after ultraviolet (UV) and microbial aging processes. Both UV aging and microbial aging significantly enhanced the adsorption levels of mesotrione on PLA and PBAT BMPs by approximately two-fold, driven by pore filling, hydrogen bonding, and it-it conjugation. The adsorption capacity of mesotrione on BMPs decreased with the pH from 3.0 to 11.0, which was involved by electrostatic interactions. In addition, salt ionic strength (Na+, Ca2+, Mg2+, Fe3+) generally inhibited the adsorption due to ions competition for adsorption sites. Notably, mesotrione exhibited high bioaccessibility when adsorbed onto BMPs, with aged BMPs exhibiting greater desorption quantities in gastrointestinal fluids compared to virgin BMPs. These findings provide effective insights into the potential health threats posed by BMPs carrying herbicides in the environment and offer applicable guidance for managing and remediating composite pollution involving BMPs and adsorbed contaminants.