Abstract: Slope stability is the pre-condition of safety and production in open-pit mine and rainfall is one of the most important factors that affect slope stability.Especially for thick alluvium open-pit slope，it is more important to study the effect of rainfall.Combining with the hydrological and geological data at eastern slope of Yanshan Iron Mine，The changes of groundwater depth，and its impact on safety factors of the slope and the distribution of the seepage field under different rainfall conditions were studied.The simulation results showed that the existed water shutoff curtain at the early construction does not work well in lowering the groundwater level and the groundwater depth is constantly low.The fine sand layer and gravel layer in the slope are rich in water，forming a coherent groundwater transport corridor，and water leakage appears in the slope.In heavy raining，slope water inflow is increased dramatically，slope safety factor decreased obviously，and Landslide may occur.Based on this and the status quo，a method for drainage was proposed in order to improve the safety factor and ensure the safety and stable production of the mine.

Key words: Seepage, Safety factor, Slope calculation, Numerical simulation