<p>The subgrade compactness often exhibits decay after initial construction and significant variability. These characteristics of compactness directly impact the hydraulic characteristics of the subgrade soil, which in turn affect the evolution and equilibrium of the subgrade humidity field. This study aimed to develop a moisture migration model considering the influence of compactness variability and decay, and reveal its influence on the evolution and equilibrium laws of the subgrade humidity field. First, a new hydraulic characteristics model was developed incorporating the compactness and overlying stress, and exhibited good predictive performance (R² = 0.93). Then, a spatiotemporal evolution equation for compactness was constructed considering the randomness and decay. Based on a finite element model, an efficient calculation method for subgrade moisture evolution was also developed considering the coupled effects of hydro-mechanical-compactness. The results indicated that the developed subgrade moisture calculation method thoroughly accounted for the compactness characteristics. Matric suction experienced a sharp decrease, with moisture content increasing from its initial state within approximately 4 years, eventually reaching a relative equilibrium. When the compactness decays by 5%, the matric suction and moisture content in the equilibrium state increased by about 5%. With the inclusion of the compactness randomness, the matric suction distribution displayed zigzag fluctuations. The equilibrium moisture content at each position followed a normal distribution with a high level of fitting accuracy (R² = 0.99). These findings can provide a theoretical basis for the probabilistic design of road structures, ensuring the long-term stability and performance of road infrastructure.</p>