In order to obtain the distribution characterization and influence factors of subgrade moisture in humid zone, four expressways were investigated by coring method or time domain reflectometry(TDR) probes. The results indicate that, the groundwater table is a main factor affecting moisture distribution in clayey subgrade beneath pavement, the equilibrium gravimetric water content is about 5% higher than optimum water content; the infiltration through the uncovered shoulder and slope can increase the subgrade moisture under the outer lane, and result in the wetting and drying cycles; the poor drainability of unpaved median also leads to the water accumulation under the inner lane; the sandy subgrade is less influenced by groundwater, and the lag between rainfall and subgrade moisture variation is shorter. So, the durability of drainage facilities should be taken into consideration in highway design, and the material parameters should be selected based on equilibrium moisture content determined by underground water, especially on the actual moisture content beneath the outer lane.