There are many examples of environmental dredging applied on the polluted lakes, reservoirs, and rivers. However, the treatment of dredging residual water and the control standards of tailwater discharge have not been clearly defined yet. To address this issue, this paper takes the environmental dredging of the Shahe Reservoir and its supporting mud plate and frame filter press dewatering project in Liyang as an example, and studies the water quality after the occurrence of residual water and the changing pattern in each treatment process. Based on this pattern, the discharge standard of dredged tailwater is discussed. The results indicate that most of the pollutants in the dredged water can be removed after the first-level flocculation treatment. The removal rates of suspended solid, total nitrogen, total phosphorus and COD are 73% to 99%, 21% to 72%, 69% to 96% and 32% to 48% respectively, but the subsequent ecological treatment is not effective. Referring to the "Integrated Wastewater Discharge Standard" (GB 8978-1996), most of the treated tail water achieve the first level discharge standard. If the discharge standard needs to be further improved, the ammonia nitrogen in the tailwater would become a restrictive indicator.