Consolidated drained triaxial shear tests, mercury injection tests and electron microscope scanning tests were conducted in this study to investigate the effects of the water content fluctuation on the mechanical properties of soil from the Gongming road and the meso-mechanism of deterioration. Based on the Duncan-Chang model, an empirical constitutive model of the subgrade soil was proposed to consider the effect of water content fluctuation. The results show that the development of pores, macropores and micro-cracks leads to the transformation of soil shear stress-strain relationship from the strain-softening to the strain-hardening, the improvement of soil shear shrinkage characteristics and the decrease of shear strength. With the increase of moisture content fluctuation, the shear shrinkage characteristics of the sample become more obvious, but the shear strength decreases. The calculation results of the proposed constitutive model for the subgrade soil are in good agreement with the test results, which proves the effectiveness and applicability of the model.