The "JC" method reliability is used to evaluate the steadiness of impervious base reasonably.Limit state equations for reliability of impervious base are given after analyzing randomness of hydraulic pressure,depth of destructive bed and coefficient of water inrush.This paper calculates reliability index, lose efficacy probability and coefficient of water inrush corresponding to the thickness of different impervious base.The lose efficacy probability of the impervious base ranges suitably in 1.5×10^-3～3.0×10^-3 by contrast with other accidents.