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Presence of microplastics (MPs) in wastewater has posed a huge ecosystem risk. Constructed wetlands (CWs) can effectively intercept MPs, while with MPs accumulation the response of CWs’ performance is still unclear. In order to evaluate those effects, we conducted a 370-day experiment using CW microcosms fed with different levels (0, 10, 100, and 1000 μg/L) of polystyrene (PS) MPs (diameter: 50− 100 μm). Results showed that nitrogen removal efficiency was increased (by 3.9%− 24.7%) during the first 60 days and then decreased (by 7.1%− 41.3%) with MPs accumulating, but no obvious change in COD and TP removal was observed. From further analysis, MPs accumulation changed the biofilm composition (TOC content increased from 41.4% to 52.7%), substrate porosity (electrical resistivity increased by 1.2–2.4 folds), and oxygen mass transfer (| K L a, O 2| increased from 3.5% to 18.6%). Moreover, the …