Abstract: Aluminum flocculants have an important role in the flocculation and settling of fine sediment, and are often used to clear muddy water and treat agricultural waste water.In this paper,sediment concentration of 10 g/L is prepared in 1 000mL cylinder which contained AlCl₃ solutions with concentration ranging from 0 to 1.7mmol/L.The relative sediment concentration at the depth of 20cm below suspension surface is measured with a 20mL pipette.The results show that the sediment concentration decreases exponentially with time at the same depth below suspension surface.Effects of electrolyte on fine sediment flocculation and settling become small and the mean velocity decreases obviously after the suspension is mixed completely for several times.At the given sediment concentration,the emergence of the interface between supernatant and sediment suspension depends on the electrolyte and its concentration.When AlCl₃ concentration is higher than 0.9mmol/L,the obvious interface will appear.At the same time,the interface settles uniformly with time,and the mean particle size is 0.0315mm with the mean velocity of 4.756cm/min.The critical particle size to flocculate is 0.027mm.