Ganzhou City in Jiangxi Province, China, serves as a critical source and supplier of rare earth ions. However, rampant mining activities have led to severe soil erosion, persistent pollution, and considerable land degradation within the mining areas. Grassed waterways (GWWs) offer substantial advantages in drainage, erosion resistance, and promoting sediment deposition. This study investigated the utilization of the GWW for soil conservation in rare earth tailings in southern Jiangxi Province, focusing on the dynamic effects of flow and slope on runoff processes, sediment transport, and particle sorting during multiple scouring events. Results demonstrate that the GWW effectively reduced runoff rates and significantly controlled sediment yield. Both flow and slope influence sediment yield rates, with slope having a more pronounced impact. Particle size distribution in GWWs, under varying flow rates, slopes, and field conditions, typically exhibits bimodal patterns, with particles in the 0–0.068 mm and 0.094–0.171 mm ranges dominating the eroded sediment. Transport mechanisms, primarily suspension/saltation (>50%) and rolling, affect these particle sizes respectively, while coarse particles (>0.955 mm) are predominantly deposited. The GWW enhanced particle separation, minimizing the loss of large particles, including agglomerates and individual particles, thereby reducing overall sediment generation. This study provides valuable insights into the potential of GWW for mitigating runoff and sediment production and optimizing deposition processes, providing practical implications for engineering applications.