Abstract: Fault F2 went through the dam foundation with angle 70°with the axis of right abutment of auxiliary dam and 90 m wide. The fault was mainly composed of mudstone and sandstone with low compressive strength and high content of soluble salt. As erosion of groundwater to portland cement, the effect of original foundation impervious curtain was declined. To prevent seepage water spread along fault F2 to downstream and affect the stability of south slope, plastic concrete cut-off wall was adopted. The paper introduced the design, construction, pre-drilling and quality control of the cut-off wall, for reference.