Abstract: The cutter suction dredger is often used in dredging of rivers and lakes. The core of this method of dredging is to extract the mixture of sediment and water after the agitating process. It usually results in large residue of high water content slurry, which must be reduced through soil-water separation process. However, the clogging problem often occurs in the process of the soil-water separation, resulting in lower efficiency of dewatering. To solve this problem, the vacuum filtration tests are carried out to explore the related factors within the separation process. These factors include particle-size distribution of the slurry, initial water content of the slurry, pore size of the geotextiles and vacuum pressure, which might explain the cause mechanism of the clogging. The results indicate that the permeability of the cake accumulated on the surface of geotextiles is the major factor influencing the soil-water separation. When the permeability of cake is low, it comes to clog, and other factors are ignored. The permeability of the cake increases due to the particle aggregation caused by the addition of the flocculants, which improve the effects of separation. What is more, the smaller flocs dominate the permeability of the cake. Therefore the parameters of the smaller flocs such as D₁₀, D₁₅ and D₂₀ will help select better flocculants and dosages, or predict the separation process of slurry. The addition of the flocculants barely alters the bound water content. Thus, when assessing the effects of soil-water separation, the bound water should not be considered.