Mountain tourism and illegal dumping have historically increased, leading to significant waste accumulation on the steep slopes of mountains. The issue of solid waste pollution in these areas has recently worsened, mainly due to extreme conditions. Extreme rainfall and landslides often carry solid waste downstream to reservoir catchment areas, indirectly contributing to secondary pollution from solid waste. While most previous studies focused on waste generation, this has investigated how waste moves in response to environmental changes. This study examines the Chiayi Fanlu area in Taiwan as a case study. First, it assessed the extent of illegal solid waste and estimated its volume using aerial photography. Second, this study integrated a Digital Terrain Model to analyze the environmental conditions and basic parameters related to the locations of solid waste. Lastly, this study developed a numerical dynamic simulation model based on the SRH-2D Model to simulate solid waste trajectory in mountainous regions during heavy, severe, and extreme rain. Our findings revealed that waste movement downstream during heavy rain was minimal. In contrast, during heavy rain scenarios, the scouring effect ranged from 10 to 30 m. In this study, the waste did not disperse significantly downstream; it remained relatively concentrated.