Abstract: Experimental studies are performed on red clay and expansive soils from Guangxi area in order to establish the relationship between shear strength parameters of swell-shrinking soils and number of wetting-drying cycles. Then, the relationship between cohesive attenuation rate and cyclic number is found. Based on the characteristics of the cohesive attenuation curve, the S-shaped function is adopted for fitting. The results show that the cohesion of swell-shrinking soils decreases with the increasing number of cycles and finally reaches a constant state. However, the friction angle keeps stable under the influence of the wetting-drying effects. Furthermore, the S-shaped function can well describe the strength attenuation law, and it overcomes the shortcomings of the previous fitting with linear function, polynomial function and exponential function because it can reflect the law that the soil strength is close to a stable value after certain wetting-drying cycles.