Abstract: The fracture evolution and shear strength index of red clay under wetting-drying cycles are analyzed by using the computer image processing and triaxial tests. The relationship between the shear strength index and the fracture density of red clay is established. The calculation parameters of the slope stability are discussed with the strength index reduction. The results show that the fracture density increases with the numbers of wetting-drying cycles, and eventually tends to be stable. The first and second wetting-drying cycles have the greatest effect on the crack development of red clay. The stress-strain of red clay is weakly hardening under wetting-drying cycles. The destruction form belongs to the Bulging damage. The wetting-drying cycle significantly reduces the shear strength of red clay, and the first attenuation is large. After a certain number of drying-wetting cycles, the intensity index tends to be stable. The cohesion of the attenuation is significantly greater than the interior angle of friction. The stability calculation parameters of the slope under the wetting-drying cycle is suggested using the value of the long-term strength index, that is, the cohesion stability is between 54% and 57% of the non-circulating value, the internal friction angle ranges from 45% to 63%. The relationship between the shear strength index and the crack density of red clay under drying-wetting cycles can be fitted by using a quadratic polynomial.