Based on our concern for local communities, we have chosen the Gaoping River basin to study the size and concentration of microplastics in the river. We use spectroscopy to understand the materials of these plastic particles. The Gaoping River basin, which spans the Kaohsiung and Pingtung regions, is the largest river in this area. We conducted water sampling during both dry and wet seasons to compare the types of microplastics present in different seasons. In terms of shape, plastic fibers were most abundant during the dry season, while plastic films were most prevalent during the wet season. Regarding size, particles measuring 100–500 μm were most common in the dry season, whereas those measuring 25–100 μm were more frequent in the wet season. As for materials, the predominant types found in the dry season were PE (polyethylene) and PP (polypropylene), while in the wet season, PE and PET (polyethylene terephthalate) were most common. In terms of color, the majority of the particles were white and transparent. The results of this study can serve as important monitoring references for relevant authorities, especially since there are few research organizations in Taiwan investigating the types and concentrations of microplastics in the Gaoping River basin. Thus, this research is a significant and pioneering contribution in Taiwan, providing valuable insights for further studies by related entities.