This study investigates the sources of microplastics (MPs) in urban water bodies in Beijing, combining MP diversity and the conditional fragmentation model. The research identifies significant sources of MPs in rivers and lakes, including suburban sewage, soils, and dry and wet deposition. The conditional fragmentation model is used to assess the fragmentation and stability of MPs, revealing that suburban sewage is a major source with extremely high abundance (520,000 items·m⁻³). The study also finds that non-fibrous polypropylene (PP) is primarily controlled by soils, while sewage sludge contributes notably to fibrous polyethylene terephthalate (PET). The results provide insights into the accurate source apportionment of MPs and contribute to a better understanding of their fate in urban environments. The study highlights the importance of managing MPs in suburban sewage and the role of atmospheric and wet deposition as diffuse pollution sources.This study investigates the sources of microplastics (MPs) in urban water bodies in Beijing, combining MP diversity and the conditional fragmentation model. The research identifies significant sources of MPs in rivers and lakes, including suburban sewage, soils, and dry and wet deposition. The conditional fragmentation model is used to assess the fragmentation and stability of MPs, revealing that suburban sewage is a major source with extremely high abundance (520,000 items·m⁻³). The study also finds that non-fibrous polypropylene (PP) is primarily controlled by soils, while sewage sludge contributes notably to fibrous polyethylene terephthalate (PET). The results provide insights into the accurate source apportionment of MPs and contribute to a better understanding of their fate in urban environments. The study highlights the importance of managing MPs in suburban sewage and the role of atmospheric and wet deposition as diffuse pollution sources.