Abstract: Microplastics in aquatic environments are colonised by microorganisms, such as bacteria and phytoplankton, forming a biofilm on their surface. This biofilm alters the properties of the microplastics, affecting their environmental behaviour. This paper investigates the bio-aging behaviour of polystyrene (PS) microplastics in the estuary area of Yongdingxin river under different salinities and their effects on the adsorption characteristics of copper ions. The aging process, adsorption kinetics and isothermal adsorption are discussed. The study found that the biofilm alters the state of microplastics in water. The adsorption process of Cu(Ⅱ) on pristine PS microplastics followed the quasi-secondary kinetic model and Langmuir isothermal model, with R² values of 0.9978 and 0.9888, respectively. While the adsorption of Cu (Ⅱ) on bio-aged microplastics followed the intra-particle diffusion model and Freundlich isothermal model, with R² values of 0.9404 and 0.9918, respectively. Bio-aging transformed the adsorption process of Cu (Ⅱ) on microplastics from monolayer adsorption, which mainly occurs on the surface of the microplastics, to multilayer adsorption and intra-particle diffusion, which occur on inhomogeneous surfaces. The bio-aged microplastics exhibited an increase in maximum Cu (Ⅱ) adsorption of 0.287 mg/g and 0.158 mg/g in distilled water and artificial seawater, respectively. This suggests that biofilms provide more adsorption sites and enhance the adsorption capacity. Additionally, biofilms cultivated in low salinity water have a stronger effect on adsorption than those in high salinity water. This paper provides a new scientific basis for investigating microplastics in the environment and their behaviour, which is significant for the ecological risk assessment of microplastics.