Abstract: A method to calculate unsteady seepage is established in this paper through studying cut off negative pressure method and extending it to 3 D unsteady seepage analysis. Some examples show that this method have advantages of strict in theory, quick in convergence and high in precision, and it is an effective method for the analysis of steady and unsteady seepage. The suitability of cut off negative pressure method to unsaturated seepage is remained to be further reseached, especially the selection of maximum negative pressure.