Abstract: The variation laws of seepage lines in upstream shell of core dams under change of reservoir water level are investigated to calculate the influence of seepage pressure on the stability of upstream dam slope. Model tests are conducted to determine the variation laws of seepage lines under a variety of ranges of slope ratio of upstream dam slope, velocity of water level drawdown and permeability coefficient of dam shell. Comparison among model tests, finite element method and empirical formula method is also presented. The results reveal that the faster the reservoir level declines, the higher the seepage lines and the highest point. The steeper the dam slope, the lower the position of seepage lines. When the permeability coefficient is up to 2.45?10^-1cm/s, the highest point of the seepage lines obtained from the finite element analysis is little higher than that of the model tests. The result obtained from the empirical method is the smallest one.