Refined study of the influence of the water level flucration on bank slope stability is of great significance. Taking the Shigenao Landslide in Baihetan Hyreopower Reservoir as an example in the paper， based on geological conditions， combined the monitoring data of universal geo-hazrad monitoring instrument， rainfall data from the meteorological station and reservoir water level fluctuation data， the stability of the slope in different water levers and water lever dropped over 20d in differente speeds was simulated by the finite element method. The results show that the safety factor increases about 17.6% in the stable seepage flow compared with the condition of transient seepage， the stability increased as the reservoir water level rises， otherwise the stability decreases. The greater the drop rate of reservoir water level， the worse the stability， and the stability decreases by about 3.2% when the develeration rate increases by 0.5m/d. The factors of stability decreases significantly in the early stage， and reached to the lowest in the 10^th day，which is highly coincide with the GNSS monitoring data. The monitoring data predicted the occurrence of the landslide in advance which shows the monitoring effect and forewarning funtion of the universal monitoring instrument.