Abstract: The unsteady seepage field caused by reservoir or river water level drawdown is complex and difficult to determine. In this paper, the physical processes of unsteady seepage and slope stability were simulated in the sandbox with silt, fine sand, clay. On the basis of the result of sandbox tests, the characteristics of the seepage field during upstream water level drawdown in front of the slopes was analyzed, the free surfaces and equipotential lines in slope seepage field with different permeability materials, slope forms and drawdown velocities of the outside water level were compared. The influences of storativity μ, permeability coefficient k and decline velocity of reservoir water level v on the maximum high point of free surface, pore water pressure in upstream slope and seepage gradient were researched. The standard to judge the sudden decline, gradual decline and quick decline of reservoir water level using k/μv was quantitatively analysed, and calculating saturation line formula was discussed. The results show that the pore pressure and infiltration gradient inside upstream slope during the water level decline are higher with smaller permeability coefficient and larger storativity inside soil slope, and the position of the maximum high point is also more high, that the seepage direction of upstream slope points to upstream slope surface, and that there exists a linear relationship between the ratio of the height of maximum point of saturation line to water depth h0/H and relative decline time t/T, and an exponential relationship between the unsteady seepage judgement index k/μv and its linear coefficient.