This study investigated the distribution, toxicity and transmission characteristics of MPs along trophic level on macrobenthos under environmental concentration in Sargassum ecosystem. At first, the abundance of MPs in Sargassum ecosystem was investigated, and the transport characteristics of MPs along trophic levels on organisms were studied by carbon and nitrogen isotope analysis. The concentration of MPs in sediments from in-situ investigations was defined as the environmental concentration in indoor experiments. Then, the microenvironment experiments were carried out based on the indoor water-organism-sediment ecosystem under environmental concentration. The distribution characteristics of MPs and impacts of MPs on physiological functions of typical macrobenthos were studied by measuring biomarkers and microbial diversity in Sargassum ecosystem. This study carried significant implication for assessing the ecological risk posed by MPs to Sargassum ecosystems.