Abstract: The stress-strain relationship of loess with different depth and water content of loess slope is studied with the reduced triaxial compress test of undisturbed loess and compared with the results of conventional triaxial compress test of undisturbed loess. It is pointed that the deformation and failure process of loess slope under excavation and unloading can be reasonably explained by the reduced triaxial compress test. The test results show that the surface unsaturated loess of loess slope with toe excavation is easy to appear strain softening or plastic flow, and strength reduction, the deep layer saturated loess appears strain softening under the condition of mid-lower confining pressure and strain hardening and strength increase under high confining pressure. Then, based on the results of reduced triaxial compress test, deformation mechanism analysis of loess cut slope is made. It is considered that the particular engineering geological conditions of loess slope leads to a disadvantageous combination of water content and confining pressure to the strength of soil. Then the disadvantageous combination causes deformation and failure of soil mass and slope. The deformation and failure mechanism of loess slope under excavation belongs to creeping slide and crack arising from pressing or retrogressive slide.