Abstract: The anti-seepage capability of the traditional capillary barrier cover in humid regions is immature. The capillary barrier cover with unsaturated drainage layer (UDL) represents a new direction of applying the capillary barrier cover in humid regions. Through the laboratory rainfall infiltration model tests, the anti-seepage mechanism of the capillary barrier cover with UDL is analyzed, and the effects of width and permeability coefficient of UDL on its drainage performance are studied. The test results show that: (1) The newly added unsaturated drainage layer has the suction function. By setting the drainage layer, the breakthrough time is extended from 730 min to 990 min, an increase of 35.6%. (2) When the drainage layer and the fine-grained layer are constructed by using the same materials, there are few effects on the suction performance by increasing the width ratio of the drainage layer to the fine-grained layer from 1:8 to 1:4. (3) Increasing the ratio of permeability coefficient between the drainage layer and the fine-grained layer can enhance its suction effects and achieve the ideal working state in which all the infiltration water is discharged by the drainage layer. Finally, a preliminary criterion for the ideal drainage state of UDL is proposed, and its analysis results are in good agreement with the test ones. This study may provide a new reference for the structural optimization and design of the capillary barrier cover in humid areas.