The Baihetan super-high arch dam is constructed under complicated geological conditions, including large-scale columnar jointed basalt (CJB) with closely spaced joints and multiple interlayer and intralayer shear belts at the lower part of dam base, which may increase a risk of seepage in adjacent dam foundations. Therefore, curtain grouting is used to reduce the permeability of dam foundations. Lugeon and grouting field tests and geological exploration were carried out to investigate curtain grouting characteristics of the dam foundation. The findings were as follows: (1) the permeability coefficient of the first type of CJB and breccia lava without unloading is 2 × 10 ~ 5 × 10 cm/s; that of the second type of CJB without unloading is 2.5 × 10 ~ 5 × 10 cm/s; and that of the unloaded third type of massive basalt is 1 × 10 ~ 3 × 10 cm/s. (2) The rock core shows that the slurry caused repeated splitting of the rock mass and widened the cracks of the rock mass, which resulted in plastic deformation of the rock mass. The P ~ Q curve of the five-point Lugeon test shows that the curve shape is turbulent and that the maximum Lugeon value appears at the highest pressure. (3) The phenomenon of low-permeability conditions with a large grout take can be frequently observed. The proportion of the hole stages with a water permeability less than 1 LU and grout take greater than 50 kg/m ranges from 12.5 to 37.5%. The proportion of holes with permeability greater than 10 Lu and grout take greater than 50 kg/m is 0 to 12.5%. The findings from this case study can provide a reference for engineering construction.