Abstract: It is a frontier technology used in the analysis of soil liquefaction effects that the liquefied soils are regarded as the fluid. In which, a key problem is how to describe the fluid characteristics of the liquefied soils. As a basic assumption, the concepts of the thixotropic fluid induced by the vibration pore water pressure are introduced to analyze fluid characteristics of the saturated sands after entering the initial fluid state. The initial fluid state is determined according to the maximum curvature of the accelerating growth phase of the flowing property curve. The flowing property curve can be obtained by employing the undrained cyclic triaxial experiments of the saturated sands, and the curvature is calculated using the point difference method. By analyzing the data from the conducted undrained cyclic triaxial experiments, it is discovered that the relationships between shear stress and shear strain rate of the saturated sands after the initial fluid state meet the state equation of the Cross thixotropic fluid. The structural parameter of the fluid is proved to be directly proportional to the residual effective stress ratio. Additionally, it is also discovered that the physical essence described by the rate equation is the growth process of the vibration pore water pressure resulting in the inner structural damage of sands. It is concluded that the proposed basic assumption is confirmed. It is also proved that the saturated sands after entering the initial fluid state possess the characteristics of the thixotropy fluid induced by the vibration pore water pressure.