In the process of reservoir water level declining，because of the adsorption of cohesive soil on water moisture，the moisture content of the cohesive soil can not dissipate in time，resulting in viscous dam body water level was higher than the reservoir water level，this phenomenon had a great impact on seepage，stability and stress of dam slope.Taking Dahe Tin reservoir as an example，based on the saturated-unsaturated seepage theory，a two-dimensional finite element method was used，the seepage stability，stress and displacement under hydrostatic load，drawdown rate of reservoir water level were analyzed，the research results play a guiding role in the operation and management of reservoirs.