Abstract: Seepage field distribution is a basis of the piping study. The piping area is limited around the piping mouth in two stratum dike foundation. The seepage field will experience a dynamic change process after the piping mouth formation. For the above mentioned reasons， an unsteady seepage model of the piping foundations combined with an unsteady flow equation and superposition principle is proposed with the piping mouth generalized as a spherical well. And then the seepage field distribution is given by analyses. Through an example， the change rules of the water head and the hydraulic gradient with time after the formation of the piping mouth are studied. And then analyses of differences between the seepage field distributions under the spherical well and the fully penetrating well are also carried out. The analysis results show that the unsteady seepage around the piping mouth as its formation is one of the most important reasons for the water and soils rushing out when the piping mouth forms. And the piping area would be erroneously judged with the results from seepage distribution formed by the fully penetrating well.