Microplastics and antibiotics not only pollute aquatic environments and threaten human health, but are also tricky to remove. Microplastics adsorb antibiotics, and, before being released into the natural environment, most microplastics pass through some wastewater treatment and/or disinfection (such as chlorination) facilities. It is therefore necessary to understand how these treatment processes may affect or alter microplastics’ properties, particularly their ability to adsorb antibiotics, and whether or not the two, when bound together, may present exacerbated harm to the environment. This study used both laboratory tests and molecular dynamics simulation to investigate the mechanism through which chlorinated microplastics (specifically polystyrene) adsorb the antibiotic tetracycline, and showed that chlorination gave the polystyrene a larger interaction area (> 21%) and more free energy (> 14%) to adsorb tetracycline. Van der Waals (vdW) forces played a more dominant role than electrostatics in facilitating tetracycline’s adsorption. Moreover, a density functional theory analysis demonstrated that the vdW potentials of the microplastics decreased as more and more hydrogen atoms became replaced by chlorine, suggesting a facilitation of the adsorption of polycyclic antibiotic molecules. The experimental results confirmed the simulation’s prediction that a higher degree of chlorination significantly increases the polystyrene’s adsorption capacity, whereas pH and salinity had almost no effect on the adsorption. This study demonstrates that disinfection elevates the risk of antibiotics adhering to and accumulating on the surface of microplastics.