Microplastics (MPs) are gradually spreading around the world and becoming a ubiquitous environmental contamination in aquatic environments. Due to its unique physicochemical properties, MPs are considered to be strong adsorbents for environmental pollutants and may affect their fate and toxicity in the environment. In this study, the adsorption behaviors of four typical heavy metal ions (Pb2+, Cu2+, Cd2+, and Ni2+) on two sizes of polystyrene MPs (10 μm and 50 μm) were investigated based on batch experiments, and the combined effects of heavy metals and MPs were assessed using Daphnia magna as model. The results showed that smaller MPs (SMPs) exhibited higher adsorption capacities for metal ions (0.261–0.579 mg/g) than that of the larger MPs (LMPs) (0.243–0.525 mg/g), and the affinity sequence of heavy metals on MPs is Pb2+ > Cu2+ > Cd2+ > Ni2+. There are better admirable agreements for pseudo-second-order model and Langmuir model to fit the adsorption kinetics and adsorption isotherms, respectively. Additionally, the combined toxicity of MPs and heavy metals was negatively correlated with the adsorption capacity between them. The combined effects of mixtures toward D. magna changed from antagonism to additive effect with increasing MPs concentrations, and SMPs exhibited higher toxicological risk than LMPs. Our findings compared the accumulative effects of various heavy metals on MPs and can contribute to understanding the combined effects of plastics and heavy metals on biota. Display omitted •Affinity sequence of heavy metals on microplastics was Pb2+ > Cu2+ > Cd2+ > Ni2+.•Combined effects of mixtures varied with increasing microplastics concentrations.•Combined toxicity of microplastics and metals was related to adsorption behaviors.•Smaller microplastics exhibited higher environmental risks.