Seepage simulation was conducted under the condition of water seal failure behind the cut off wall in Jinping 2nd Stage Hydropower Station. The effects of the failure of water seal on seepage discharge, porous pressure and seepage stability were analyzed. The calculated result shows that the seepage control effect of the cutoff wall is lost when the water seal is destroyed, and then uplift pressures at apron and the seepage discharge increase considerably. The apron may be destroyed without drainage measure and seepage damage occurs. However, if the fracture of water seal is filled with silts, in which permeability coefficient is a 10 -3 cm/s magnitude, the hydraulic characteristics are similar to that under normal condition.