As a new type of pollutant, microplastics have attracted increased attention because of their widespread and persistent existence in the water environment. In this study, we investigated one of the eight largest lakes in China—Wuliangsuhai Lake in Inner Mongolia. The microplastic concentration in Wuliangsuhai Lake ranges from 3.12 to 11.25 n/L. The different functional areas in Wuliangsuhai can be divided into four categories, namely effluent, fishing, intake and wetland areas. The highest microplastic concentration occurs in the intake area. The collected microplastics are divided into four types, i.e., fibers, pellets, fragments and films, of which fibers are the most abundant. Among the four functional areas of Wuliangsuhai Lake, the highest fiber concentration is observed in the fishing area. The microplastics in Wuliangsuhai Lake are mainly small size, and the microplastics smaller than 2 mm account for 98.2% of the total microplastics, while the microplastic size in the intake area is relatively large. Polystyrene and polyethylene are the main polymer types. Agricultural wastewater, domestic sewage and fishery discharge have the greatest impact on the microplastic distribution. This study reveals that the inland lakes in northern China are polluted by microplastics, which may cause potential harm to the surrounding environment. Display omitted •Microplastics occurrence in Wuliangsuhai Lake were investigated.•The dominant microplastics were polystyrene fiber with 0–2 mm in size.•The discharge of human sewage and industrial wastewater were the main source of microplastics.•The spatial distribution of microplastics varies between different functional areas.