This paper studies the mechanism of landslide instability of accumulation layer under reservoir earthquake. Taking Yaquewan landslide as an example, a numerical model was established based on the coupling of Quake/W and Slope/W in Geo-Studio to analyze its stability change and dynamic response under the action of horizontal earthquake. It is concluded that the stability coefficient of the landslide of this accumulation layer fluctuates between 1.199~0.859 with time under the action of horizontal earthquake. As the reservoir water level decreases from 175m to 145m, the stability of the landslide decreases from unstable to unstable. The thicker the accumulation layer is, the larger the displacement is after the earthquake, and the displacement of the sliding zone and the bedrock at the back edge is consistent. The peak acceleration of monitoring point increases with the increase of elevation, and the time of peak acceleration also lags with the increase of elevation. The peak acceleration of the slope is affected by the slope. When the slope of the monitoring point changes greatly, the peak acceleration will decrease accordingly. Analysis of the instability mechanism of under horizontal seismic action, it is concluded that shear damage accumulation landslide trailing edge, the surface rock mass along the parallel to the sliding surface sliding of the sliding zone, cut exports for water flooded area, landslide from the surface to the deep sliding, eventually overlaying soil sliding damage, landslide slip at the same time can lead to trailing edge bedrock airport surface, easy to become a collapse.