During the construction and operation of reservoir, the repeated rise and fall of water level in the reservoir affects the safety of bank slope. The deterioration law of the mechanics properties of rock mass of bank slope under cyclic water saturation and drying was investigated in this research via triaxial compression test of sandstone in water level fluctuation zone. The concept of deterioration rate was introduced to explore the effect of water-rock interaction on sandstone's mechanical properties. And the deterioration mechanism of sandstone under water-rock interaction was analyzed by means of scanning electron microscope. Results demonstrated that the peak compressive strength of sandstone increased with the climbing of confining pressure under the same degree of water-rock interaction; under the same confining pressure, the peak compressive strength, cohesion, and internal friction angle all decreased with the proceeding of the wet-dry cycles, and such relationship accorded with exponential distribution. When the interaction between water and rock intensified, the minimum value of the deterioration rate of peak compressive strength tended towards 31.35%; cohesion, 30.61%; and internal friction angle, 15.37%. The fluctuation of water level had great influence on peak compressive strength and cohesion of sandstone. The relations between the deterioration rates of peak compressive strength, cohesion, and internal friction angle and the cycle number N also obeyed exponential distribution. Last but not the least, the mechanism of rock deterioration under water-rock interaction is studied based on the triaxial compression test result.