Abstract: The Outang landslide is a giant complex ancient landslide in the Three Gorges Reservoir Region. It consists of multiple sliding zones and multiple sliding masses. Some obvious creeping slip of the landslide were found after the impoundment of the Three Gorges Reservoir. The landslide has a great threat to thousands of people's lives and properties nearby. In order to clarify the response characteristics and the failure mechanism of the landslide under the influence of water level fluctuation, a model test is done. The model test material development and the accurate control of the water level fluctuation can simulate the process of instability of the Outang landslide. Response characteristics and mechanical mechanism of the Outang landslide are discussed. Results indicate that the fast the speed of the water level fluctuation, the bigger deformation of the three phase sliding masses which compose Outang landslide. The influence of water level fluctuation is concentrated in the zone of the first phase mass. The rapid drawdown of the water level result in deformation of the slope, which makes the retrogressive sliding possibility of Outang landslide larger. The stabilities of the second phase mass and the third phase mass decline after the deformation of the first phase mass. Outang landslide is the hydrodynamic pressure landslide. The whole Outang landslide mass is less likely to destroy along the bed rock surface under water level fluctuation. The major failure mode of the Outang landslide under the water level fluctuation is the type of local buckling failure in the leading edge of the first phase sliding mass. This test reveals response characteristics and instability mechanism of the landslide under water level fluctuation, which provides reference for researching this kind of landslide.