Abstract: Clayey loess is sensitive to frequent and sudden change in water content caused by precipitation and drought, the mechanical properties deterioration is therefore serious. This paper focuses on strength deterioration of the clay loess subjected to wetting-drying (WD) cycles based on triaxial shear tests. The macroscopic swell-shrink feature and crack development for both confined and unconfined samples were measured and recorded. The changes in pore structure and mineral composition were analyzed using mercury intrusion method (MIP) and X-ray diffraction (XRD) to explore the strength deterioration mechanism. The results indicate that WD cycles induced loess strength deterioration, particularly, the deterioration enhanced under high confining pressure. The internal friction angle decreased with the cycle number increasing, while the cohesion slightly decreased at first and then increased greatly. At the microscopic scale, WD cycles dispersed the clay aggregates in loess soils and increased the clay content as well as the hydrophilicity of clay minerals. For the unconfined loess samples, the void ratio increased with the cycle number increasing, the pore size totally increased and the pore structure became looser. While when the loess samples were confined by cutting ring, the void ratio changed slightly without regularity, the volume percentage of small pores (2~8 μm) decreased obviously. The above two types of change in pore structure were consistent with the macroscopic deformation and crack development of the loess samples with and without lateral restraint. The deterioration of loess strength is controlled by the grain-size distribution, cementation between particles, pore structure and some other factors. The increase of clay content caused by WD cycles enhanced the cementation between particles, while the discretization of clay aggregates and loess pore structure weakened the occluding friction among particles. In the future work, the change in clay mineral composition and property with WD cycles should be paid attention to when addressing macroscopic behaviours, in addition, the actual confined or unconfined condition should be considered because the pore structure showed different responses to WD cycles.