The pot cover effect refers to the phenomenon in which an impermeable cover layer impedes soil moisture evaporation, leading to water accumulation beneath the cover. This effect significantly increases the moisture content of shallow soil layers, deteriorates the mechanical properties of the subgrade, and consequently induces pavement structural damage. The combination of an arid climate and pronounced temperature gradients in Gansu Province, China, which spans the north temperate arid to warm temperate humid zones, leads to a particularly prominent pot cover effect. Using an established water-vapor-heat coupling model, this study compared the formation mechanisms of the subgrade pot cover effect in Zhangye (cold arid), Lanzhou (semiarid), and Longnan (warm humid) and analyzed the effect variations caused by three fill materials: silt, sand, and clay. The evidence from this research indicates a clear dependence of the subgrade pot cover effect on climatic conditions, with its severity increasing under lower mean annual temperatures and wider annual temperature ranges. This climatic influence is evident in the regional comparison, where Zhangye experiences a more pronounced effect than Lanzhou and Longnan, culminating in a surface moisture content of 32% after 20 years, compared with 24.7% and 22.05% in the other two regions, respectively. Furthermore, the spatial distribution of moisture accumulation was found to be soil-specific, with silt subgrades accumulating more surface moisture than their sand or clay counterparts. The primary mechanism driving this accumulation is vapor migration, which reverses direction between seasons: upward toward the near-surface during freezing periods and downward into deeper layers during thawing. These findings offer valuable insights for the understanding and mitigation of the pot cover effect in subgrade engineering.