<p>Traditional soil subgrades are prone to stability degradation under long-term wet–dry cycling, where repeated moisture fluctuations induce microstructural damage and consequently reduce soil strength. To mitigate this stability degradation, this study proposes a composite layered subgrade structure with enhanced drainage performance. By incorporating a graded gravel drainage layer with a permeability coefficient multiple orders of magnitude higher than that of soil, a multi-tiered water diversion and drainage synergy system was established. Based on the Tianqiong Expressway project in Qionglai, Sichuan, field immersion tests were conducted with embedded temperature-humidity sensors and settlement plates. Combined with data from a distributed sensor network, the spatiotemporal evolution of volumetric water content across structural layers during immersion-drainage cycles was systematically investigated. Experimental results demonstrate: the layered structure stabilizes Subgrade moisture within an optimal range, whereas soil moisture in non-drained foundations increased from 28.1 to 41.6%, confirming severe water retention; The graded gravel layer enhances lateral drainage, reducing upper soil moisture through hydraulic gradient optimization and seepage path multiplication, achieving rapid water redistribution and stabilizing subgrade humidity. These findings elucidate the regulatory mechanism of composite layered structures on subgrade water retention states, providing critical design parameters for drainage layers in highway subgrade engineering. The proposed system significantly improves the long-term service performance of road infrastructure in humid, rainfall-intensive regions.</p>