Abstract: Traditional surface hardening and impermeabilization measures for earthen slopes are prone to desiccation cracking due to rainfall infiltration, threatening engineering safety. The application of an unsaturated barrier layer (CBL) for protection can enhance the safety and stability of the project. However, its hydraulic conductivity decays with the long-term process of rainfall infiltration. To address the attenuation of capillary barrier strength in conventional unsaturated CBL during rainfall infiltration, this study proposes a method to enhance the infiltration resistance of the unsaturated barrier by pressurizing the coarse-grained layer from a coupled water-gas perspective. A series of physical model tests were conducted to investigate the effects of different rainfall intensities, initial CBL moisture contents, and coarse-grained layer pressurization levels on infiltration resistance. The main findings are as follows: (1) When the coarse-grained layer is pressurized (1~3kPa), rainfall is entirely discharged along the interface between the fine-grained layer, the transition layer, and the coarse-grained layer, completely preventing rainfall breakthrough into the coarse-grained layer and significantly improving the barrier efficiency of the unsaturated CBL. (2) The steady-state lateral drainage rate of the binary structure increases with higher pressurization in the coarse-grained layer but decreases with increasing moisture content in the fine-grained layer. (3) A Comprehensive Barrier Efficacy Index (CBEI) is proposed to quantify the effectiveness of the unsaturated barrier under air injection conditions. This result innovatively establishes the soil gas phase as the drainage driving force in barrier layers, providing a scientific basis for slope protection engineering.