The determination of probability distribution of seepage parameters is a basis of reliability analysis of levee seepage stability. The research development of soil permeability and critical hydraulic gradient is presented. The probability distribution of levee soil permeability is analyzed by maximum likelihood estimation of distribution parameters and A-D test method for distribution fitting. On the basis of Terzaghi formula of flowing soil, the probability density function of critical hydraulic gradient is derived with soil relative density and void ratio as random variables. Results show that the distributions of the permeability of silty clay and silty loam are both subject to lognormal distribution, the soil critical hydraulic gradient obeys approximately normal distribution.