Abstract: In the soil-geotextile filtration system, a large amount of fine particles will migrate under the drag forces exerted by the flow if the rail is internally unstable. A layer of thin filter cake is prone to be formed between the interface of soils and geotextiles. The thickness and permeability coefficient of the cake become the important factors affecting the fihration effect. Based on the theory of seepage and sediment in the porous medium, the relations between the cake's thickness and other factors such as geotextiles thickness and permeability coefficient, outside pressure, fine particles content are studied during the migration of fine particles in the process of unstable soils' filtration. The analytical expressions are given.