In freshwater ecosystems, microplastics (MPs) are commonly found in reservoirs. However, limited information is available on the distribution of MPs in the reservoirs. In this study, we investigated the horizontal and vertical distribution characteristics of MPs in the Guanyingyan reservoir (the upper reaches of the Yangtze River, China) after impoundment and the influence of free-floating plant residues on the distribution of MPs. Results indicated that the MPs abundance in the horizontal distribution of the reservoir decreased significantly while the distance from the dam increased. The abundance of MPs in shoreline waters (average: 8.45 items L−1) was significantly higher than that in central waters (average: 4.80 items L−1). As for the vertical distribution, the percentages of fibers in the three water layers (surface, intermediate, and deep) have less variation when compared to other types of MPs. Besides, MPs who are less than 0.5 mm in size are the majority. With deeper underwater, there would be more MPs with particles smaller than 200 μm in size. At the same time, there would be fewer MPs with particles ranging from 200 to 500 μm in size. PS, PP, and PE are the main polymer types of surface water, while PVC, PE, and PET are the common type in deep water. In shoreline water, the dry weight of floating plant residues showed a positive correlation with microplastic abundance in different layers. As above said, this study confirmed that MPs in reservoirs after impoundment would tend to accumulate in the front section of the reservoir and the shoreline water. Besides, free-floating plant residues would accumulate in reservoirs, resulting in the sinking of MPs. Display omitted •Dam reservoirs are potential enrichment zones for MPs.•Microplastics accumulate in the shoreline waters of a reservoir.•PS, PP, and PE are the most frequent MPs types found on the surface of a reservoir.•Free-floating plant residues in reservoirs lead to an accumulation of MPs.