Outdoor model test was carried out for two years in humid area of China in order to study the long-term performance of the capillary-barrier cover with an unsaturated drainage layer (UDL). The size of model was 2.0 m (length) ×1.0 m (width) ×0.55 m (thickness); meanwhile, the slope rate and gradient were 1∶3 and 18.4° , respectively. From top to bottom, the structural profiles were vegetation growth layer (15 cm), silt layer (20 cm), sand layer (10 cm) and gravel layer (10 cm).The monitoring results show that: with the annual rainfall of 3 448.4 mm, the percolation is 2.62 mm, which is in line with the impervious standard of soil cover layer in humid climate zone of North America. Lateral diversion is 581.77 mm, which accounts for 16.87% of the cumulative rainfall and the lateral diversion effect is obvious. From April to November in humid area of China, the three peaks of temperature, rainfall and evapotranspiration coincide every year. The favorable climatic conditions in the same period of rain and heat, and the lateral diversion of UDL, effectively control the percolation. The continuous heavy rainfall from June to September is high risk to percolation meteorological event which is a controlled meteorological condition for seepage prevention design. From December to March of next year with scarce rainfall and weak evapotranspiration, the lateral diversion effect is obvious and avoid to percolation. The capillary barrier cover with unsaturated drainage layer is made up of silt, sand and gravel. Swelling, shrinkage and cracking are not obvious.