Abstract: Structure features and failure mechanism of soil were analyzed,equation for limiting statebased on the filter theory was abtained. Method for calculating the reasonable linear length of the ele-ment which represents the particle of soil and its stability were discussed.Through the investigation onthe randomness of homogeneous soil,the probability distribution of characteristic particle of cohesion-less soil and its characteristic values were obtained. calculation method of reliabilty and specific acci-dent on the element filter and enfire filter protecting soil were given. Reliability and specific accident ofthe filter(transition layer)proctecting the soil cushion course of the concrete face in Xibeikou CFRDwere illustraled.