In the homogeneous earth dam design, reservoir water level sudden drawdown is often a control condition of the upstream slope. The judgment and determination of the wetting front location will provide a basis for dam stability calculation during reservoir water level lowering and a reasonably judging lowering rate of water level is a key step for calculating wetting front. The k/μv represents the rate of reservoir water level being relative to seepage free surface decline, which is used for distinguishing different drops of water level： gradual drop, quick drop or sudden drop, and from this value, the saturation line can be calculated. The permeability coefficient k can be given precisely by the test；the storativity μ is a key parameter to determine the saturation line location, but there is no perfect method to identify μ, and then this paper uses a suitable contrast value obtained from the formula calculation and relative table；the rate v to determine the pace of water level decline is given by different formulas. So, an issue on the reasonable judgment of damtype and the rational choice of stability calculation indexes during water level sudden drop is proposed.