Abstract: Based on the dynamic triaxial tests on the saturated Nanjing fine sand, an obvious phenomenon is discovered that the relationship curves of the shear stress-strain rate are altered from an elliptical shape to a dumbbell one with the build-up of the pore water pressure. The relationship curve with the dumbbell shape demonstrates that the saturated sand possesses low shear resistance and fluidity. The average flow coefficient and the flow curve describing the fluidity of the saturated sand under cyclic loading are defined. It can be found apparently that the three-phase characteristics exist in the flow curves. The influences of the relative dense, effective consolidation pressure and cyclic stress ratio on the flow curves are discussed. It is presented that the initial sharply-increasing point for the average flow coefficient is taken as the critical point of the saturated sand changing from the solid state to the fluid one. The pore water pressure ratio corresponding to the initial sharply-increasing point is defined as the pore water pressure ratio of phase change. The tests show that the relationship curves of the average flow coefficient and the pore water pressure ratio under various loading conditions are similar in shape. In addition, the pore water pressure ratios of phase change for the saturated Nanjing fine sand under various test conditions are about 0.8. The relative dense, effective consolidation pressure and cyclic stress ratio have few influences on the pore water pressure ratios of phase change.