Abstract: The interlayer staggered zones in the basalt are widely distributed in the Baihetan hydropower project area in Jinsha River Basin, China. They are filled with gravel, debris, and mud. The mud is easy to be destructed. After water impoundment, the seepage gradient in the interlayer staggered zones would increase with the effects of the high water head difference and seepage control measures. Seepage failure can happen in the zones. We establish a 3D finite element fine model of the left bank of the dam area. We use a method of interpolation from different sections to determine water distribution in mountain boundary without observation data. The results show that the existing seepage control measures have effectively control the seepage pressure. The hydraulic gradient in some areas of the interlayer shear weakness zones C₂ is greater than the allowable gradient, and seepage failure can happen in these areas. The seepage-prevention hole and the reinforced curtain are effective preventions to decrease gradient, which can block reservoir water better. Moreover, the preventions can block the reservoir water to some extent when the curtain locally fails, which can ensure the seepage stability around the underground powerhouse.