Capillary barrier covers (CBCs) applied in slope protection pose a significant risk of percolation in humid regions. This study proposes a novel unsaturated drainage layer (UDL) and corresponding design criterion for CBCs with UDL in slope protection. First, a physical model test was conducted to analyze the mechanisms and influencing factors of the UDL performance under heavy rainfall. Second, the applicability of CBC with UDL in slope protection was numerically investigated using a 3-m slope. Third, a preliminary design criterion for the UDL was proposed and verified. The major findings are as follows: (1) The newly added UDL has a drainage effect, which can discharge the infiltrating water retained in the fine layer, thus preventing the percolation of CBC. (2) The saturated permeability coefficient of the UDL has a critical value, which is essential for achieving an ideal drainage state. In this state, the infiltration flux can be freely drained through the UDL, and there is no risk of breakthrough. (3) The air-entry value of the UDL should exceed the residual value of the coarse layer, and the saturated permeability coefficient of UDL is recommended as 100 times that of the fine layer. Overall, this research proves that a CBC with UDL can provide adequate slope protection through appropriate design and well-selected material. It has the potential for application in slope and landfill protection in humid regions.