Large hydro-fluctuation and periodical water level changes in the reservoir have caused the slip zone soil of the banks to experience drying-wetting cycles

which has a great influence on strength characteristics of soil. In this paper

a series of drying-wetting cycle tests on the slip zone soil from an ancient landslide is carried out. The soil samples after different runs of drying-wetting tests are then used to conduct the ring shear tests

scanning electron microscope（SEM） tests and nuclear magnetic resonance（NMR） tests. The results of these tests can be used to analyze the changes of soil microstructure and strength properties during drying-wetting cycles

and to investigate the microscopic mechanism of changes in strength properties. The research indicates that the deterioration of the residual strength of soil is obvious in drying-wetting cycles. The first three drying-wetting cycles result in a larger attenuation of soil strength

then the decay trend weakens and the soil strength gradually stabilizes. At the same time

the deterioration effect of cohesion is greater than the internal friction angle. Besides

the results also show that the clump-like aggregates are gradually disintegrated with the increase of drying-wetting cycles

and the way of connection of soil particles changes from plane-plane to plane-edge and plane-angle. Meanwhile

under the drying-wetting cycles

the number of pores in the soil increases

the morphology of the soil changes

the distance between the grains increases

and the small pores gradually change to the large pores in soil. The pore and cement microstructure changes of soil particles caused by clay mineral swelling and shrinkage are the causes of deterioration of the residual strength of slip soil under the drying-wetting cycles.