To explore the influence of hydraulic parameters of the drainage layer (UDL) on the anti-seepage performance of the capillary barrier cover (CBC) layer, the VG model was adopted to fit the soil-water characteristic curve and permeability function of soil based on the principle of unsaturated seepage in this paper. A simplified model of “fine-UDL-coarse”-type CBC layer containing a UDL was established using Geo Studio finite element software. By carrying out the numerical simulation of seepage of expansive soil slope treated with the cover layer under heavy rain conditions, the influence rules of different hydraulic parameters of the UDL on the anti-seepage performance of the CBC layer were systematically analyzed, and the reasonable hydraulic parameter values of the UDL were determined. The results show that the optimal values of hydraulic parameter n, saturated permeability coefficient k_s, and air-entry value a of the UDL are 3.5, 5.79 × 10^?4 m/s, and 9 kPa, respectively. For the UDL, increasing n, k_s, and a can all effectively extend the effective drainage length of the UDL under long-term rainfall conditions, reduce the volume of water breaking through from the sand layer to the gravel layer, and significantly reduce the range of the saturated zone of expansive soil. Therefore, when the “UDL CBC”-type cover layer is adopted in practical engineering applications, various hydraulic parameters of the UDL should be comprehensively coordinated to improve the anti-seepage performance of its CBC layer. The research results can provide a reference basis for material selection in the design of expansive soil treatment engineering using the CBC layer.