Abstract: Correctly evaluating the property evolution of saturated sand in the liquefaction process is the key to solving the problem of large deformation of liquefiable soil. The undrained triaxial tests of saturated sand were conducted to study the relationship between shear stress and shear strain, the excess pore water pressure development rate and, the growth of liquidity during the liquefaction. The investigation results showed that the transformation process of saturated sand from solid phase to liquid phase displayed a significant stage characteristic. The liquefaction process of saturated sand could be divided into four stages: solid stage, solid-liquid transition stage, thixotropic fluid stage and, stable fluid stage according to pore pressure ratio development rate which is related to the residual shear strain of soil. Confining pressure and cyclic stress ratio would affect the duration of each stage in the process of soil liquefaction. The lower the confining pressure and the higher the cyclic stress ratio, the easier for saturated sand to transform from solid stage to fluid stage. A linear relationship between the required cycles of saturated Nanjing fine sand from one stage to another and the corresponding pore pressure ratio was found in this study.