Abstract: Based on the actual state of the soil along rail transit in hot and rainy area, the resonant column tests on the soil under wetting and drying circles are conducted to study the stiffness characteristics of residual soil at small strain under hot and rainy climate. The results show that the decay law of G_max in small strain range of 10^-6~10^-4 of residual soil is quite different from the strength characteristic of clay in conventional strain range, and the maximum attenuation occurs in the metaphase of the process of wetting and drying circles. After multiple cycles, the G- curves of residual soil under low confining pressure exhibit the linear decrease tendency and sudden drawdown. The martin-Davidenkov model and the empirical relationship may reflect the relationship between and G/G_max as well as D, after wetting and drying circles, polarization phenomenon of the relationship between G/G_max and presents gradually with the increasing confining pressure. The wetting and drying circle effect of the stiffness characteristics of residual soil at small strain is explained based on the microscopic analysis, and the mechanical behavior of stiffness of residual soil in hot and rainy area results from the injury of the structural strength and microstructure remodeling. This study may provide technical reference for the selection of the dynamic parameters of the rail transit engineering, the design and construction of the similar engineering sites and the seismic analysis.