Abstract: After piping channel appears, the length of the channel may elongate or remains stable. The traditional method is to calculate the hydraulic gradient between river and piping port, and it is not precise enough. In fact, the hydraulic gradient at the tip of the channel determines the results. Considering the effects of the channel expansion, the stability of particles at the tip of the channel is analyzed, and change of the hydraulic gradient at the channel tip after channel elongation is also investigated. If the hydraulic gradient continues to grow with the channel elongation, the channel will be extended. If the hydraulic gradient keeps steady, the channel will be stable. The finite element program is compiled, to simulate the cross-section expansion and up-tracking of the channel. The program speed is accelerated by the equivalent substitution of the channel section for concentrated seepage. The model tests are carried out to observe the development of the piping channel. By comparing with the laboratory test data, it is proved that the theoretical method and the finite element program are feasible and usable.