Microplastics pollution in river systems has generated great concern; however, few studies have focused on the contributions of multiple influencing factors to microplastics in river systems. In the current study, we utilized data on microplastics in surface sediments from the Haihe River, a seagoing river in northern China to establish a generalized additive model (GAM) for quantifying the effects of multiple factors on the distribution of microplastics. A high abundance of microplastics (4980±2462 items∙kg−1 dry weight) was found. Small particles (< 1000 μm) accounted for a dominant proportion (44.8%-61.0%). Polyethylene (PE) was the chief component with an averaged fraction of 49.3%, in which low- and high-density polyethylene contributed 90.7% and 9.3% of the PE, respectively. Microplastics abundance was positively correlated with sediment TOC and the silt fraction (p < 0.05) but negatively correlated with the sand fraction (p < 0.05). The GAM could explain approximate 60% of the total microplastics abundance, and dam (28.5%), sediment TOC (22.9%), and sewage effluent (17.6%) were the main contributors to total variations in microplastics abundance. Local sewage effluent acted as an important point source of microplastics discharge, and the dam on the river greatly affected the deposition and accumulation of microplastics.