Abstract: Piping is one of the main reasons of dike and dam break in defective reservoirs. Traditional seepage theories focus on piping mechanism, developing process and controlling measures, yet seldom shed light on quantitative discrimination and nonlinear characteristics of overall piping process. Groups of laboratory tests of piping process in different grading sandy gravels were conducted, and the judging method of overall piping process was proposed by the analysis of fluid regime changes based on the Reynolds number, which shows that fines content and uniformity are main influence factors of piping failure. The whole process of piping in sandy gravels can be quantitatively divided into four stages, that is, incubation (Re<0.85), formation (0.85≤Re≤5.00), evolution (5.00<Re≤50.00) and destruction (Re>50.00). During incubation and formation stages, removable fine particles are started up and adjusted, and the relationship between hydraulic gradient and seepage velocity is linear. Viscous drag force plays an important role, and seepage flow conforms to the Darcy’s law. However, during evolution and destruction stages, seepage pathway is gradually formed and developed, and removable fine particles run off rapidly. Meanwhile, the seepage velocity changes increasingly, and the relationship between hydraulic gradient and seepage velocity is nonlinear, which shows the inertial force plays a leading role. Laminar flow gradually transfers into turbulent flow, which can be described by a quadratic equation. The research findings can provide the base for piping scientific forecast and emergency disposal.