<p>Sunken green spaces are a sustainable approach to managing urban stormwater runoff, but their unique hydrological conditions mean that environmental studies conducted in other settings are not easily transferable. Microplastics (MPs) and trace elements (TEs) are commonly coexisting pollutants in stormwater runoff, and their interaction poses a potential ecological risk to sunken green spaces. This study examined how the particle size of polypropylene (PP) MPs affects complexation with TEs and the resulting ecological risk to sunken green spaces. Laboratory experiments demonstrated that complexation between MPs and TEs reached equilibrium within 90&#xa0;min. The degree of complexation increased with the pH. MPs with a particle size of 10&#xa0;µm demonstrating the highest TE adsorption capacity while the presence of sodium chloride suppressed complexation. Among the TEs studied, the MPs had the highest affinity with Zn followed by Pb and then Cd. In the simulation experiment, the removal rate of Cd²⁺ can reach 98% or more. At the same time, the proportion of Cd²⁺ in each soil layer is more than 98%, and the accumulation of Cd²⁺ decreases with the increase of soil depth. The system containing MPs has a higher ecological risk index than the control system, which increases the overall composite risk index by 8.29%. The results show that the complexation between MPs and TEs will gradually increase the risk level of sunken green space, and it is necessary to strengthen the research and environmental management of composite pollutants.</p>