Affected by global climate change and human activities, extreme weather events such as drought and rainfall have led to landslides and other disasters in the reservoir dam. At the same time, the impact of evaporation and rainfall on the seepage and stability of the dam slope has not been considered in the design specifications for roller compacted earth-rockfill dams. Taking an earth-rockfill dam as the research object, a finite element calculation model was established to study the impacts of evaporation, rainfall and reservoir water level changes on the pore water pressure, saturation line and safety factor of anti-sliding stability of the earth-rockfill dam. And the cause mechanism of landslide was analyzed in combination with geological exploration and monitoring data. The results show that evaporation and rainfall have a great impact on the pore water pressure of the soil within 0~2 m of the downstream dam slope. The difference between the measured water level of the piezometer and the saturation line calculated by the finite element method is less than 0.4 m. At the initial stage, evaporation improves the stability of the dam slope, but evaporation causes the surface fill of the dam body to shrink and crack continuously, providing a channel for rainfall infiltration. With the increase of the number of rainfall and evaporation cycles and the rising of the reservoir water level, the cracks continue to increase, and the local stagnant areas near the bottom of the cracks are interconnected to form a saturation zone, which leads to the instability and damage of the shallow dam slope downstream of the dam. The research results provide a scientific basis for improving the design specifications of earth-rockfill dams and the early warning of reservoir dam landslides.