Micro/nanoplastics (M/NPs) have become prevalent in aquatic environments due to their widespread applications. Likewise, ubiquitous ecological macromolecules can adsorb onto M/NPs to form an “eco-corona”, which significantly alters their environmental behaviors including aggregation dynamics, adsorption/desorption, and bioavailability. Therefore, it is necessary to analyze the role of eco-corona in assessing the environmental risks of M/NPs. This review systematically summarizes the formation mechanisms of eco-corona and evaluates its regulatory effects on the stability and ecotoxicity of M/NPs. Compared with other ecological macromolecules (e.g., natural organic matter and extracellular polymeric substances), humic acid (HA) tightly binds to M/NPs through electrostatic and hydrophobic interactions, significantly affecting their hetero-aggregation behavior and colloidal stability. In terms of bioavailability, the various functional groups on the HA surface can regulate the surface charge and hydrophobicity of M/NPs, thereby affecting their bioaccumulation and “Trojan horse” effect. Notably, the HA corona alleviates M/NPs-induced growth inhibition and oxidative stress. Genotoxicity assessment further showed that HA corona can regulate the expression of genes related to oxidative stress response and detoxification pathways. Future studies should focus on the synergistic effects between eco-corona and co-existing pollutants in complex aquatic environments to elucidate the long-term ecological risks associated with eco-corona formation.