Flood-induced piping and subsequent formation of sand boils can cause severe damage to river levees. And it is known that no piping failure occurs, without seepage exit. The paper represents an experimental study on the process and mechanism of seepage exit forming on top stratum at a weak zone in a double-layer dike foundation, using sandbox modeling. According to the practical observation, three series experiments were designed. The size, thickness and the physical and mechanical properties of the top stratum are considered. The results show that the failure mode is close related to the size of the weak zone. And the hydraulic gradient of seepage exit forming is greater than the value, computed by Terzaghi’s formula.