Abstract: Reasonable evaluation of the property evolution of saturated sand during liquefaction and the dynamic response under cyclic loading is the key to solve the problem of large deformation of liquefied sand. Through the undrained cyclic triaxial tests on saturated sand, the dynamic response of saturated medium-density Nanjing fine sand during liquefaction process is analyzed, and its stage characteristics are discussed. The Gompertz function is introduced to describe the relationship between apparent viscosity and pore pressure ratio, and an improved thixotropic pore pressure fluid model is proposed. The main conclusions are as follows : (1) According to the growth rate of pore pressure ratio, the liquefaction process is divided into four stages: solid stage, solid-fluid stage, thixotropic fluid stage and stable stage. (2) The relationship between apparent viscosity and pore pressure ratio is described by using the Gompertz function instead of the linear function. A modified thixotropic pore pressure fluid model considering stage characteristics is proposed, which provides a new unified method for solving seismic liquefaction problems.