On the basis of seepage deformation test of silty clay from 16 levees in the middle and lower reaches of Yangtze River , the probability distributions of critical gradient are analyzed respectively from all data collected and data of single levee by K-S test method. Information spread method is also adopted to estimate probability density function of critical gradient of single levee with small samples. Results show that probability distribution form of vertical critical gradient of silty clay obeys the lognormal distribution, and by verification of K-S test, information spread method can be used in statistical analysis of probability distribution of stochastic parameters. Research results could offer the base for reliability analysis of levee seepage stability.