The geological hazards of landslide instability in the Three Gorges Reservoir Area are catching increasingly more attention from scholars. Water level changes are an important factor inducing landslide instability. By taking the Erdaohe Landslide in the Three Gorges Reservoir Area as the research object, based on the unsaturated fluid-solid coupling numerical method, four operation conditions for water level fluctuation were designed according to the water level fluctuation in the Three Gorges Reservoir Area. Meanwhile,the simulation computation software FLAC3D was adopted to discuss the seepage characteristics,displacement response, plastic failure, and stability evolution law of landslides in the reservoir area,with the landslide instability mechanism deeply explored. The results show that the research object of this paper is the typical hydrodynamic pressure lag type landslide, the water level rising period and the low water level constant period are favorable to the landslide, and the high water level constant period and the water level falling period are unfavorable for the landslide. Additionally,the safety factor change experiences four stages of increasing sharply,decreasing rapidly,decreasing dramatically,and slowly increasing with the water level variation.The rapid water level drop causes sudden displacement variation of the sliding body,and attention should be paid to the protection and reinforcement of the front edge of the landslide.The research results can provide a scientific basis for instability mechanism analysis of accumulation landslides and landslide control.