Abstract: The dam seepage is nonsteady due to the change of the upstream water level in actual operation. Because of its complicated boundary conditions, the determination of parameters of non steady seepage is very difficult. The steady seepage method has still been used in the current analysis of observed seepage data. Based on the theory of the non-steady seepage and the existing results of simple boundary, a new approximation model for the nonsteady seepage is proposed. Using the existing observed piezometric data, the approximate solution of the semi-theoretical and semi-empirical formula for the non steady seepage is obtained, and the calculation method for the daily change of the piezometric level under the daily change of the upstream water level. Case studies are in good agreement with the observations. Differences between calculated and observed values are very small, typically only a few centimeters, and the results are satisfactory. Calculating approximate potential values and comparing changes in different years can be used as evaluation indices of engineering seepage control capacity and its changes and can be applied to dam safety monitoring evaluation.