Abstract: Undrained triaxial tests are carried out for saturated Nanjing fine sand subjected to uniform cyclic loading. The curve shapes of the excess pore water pressure (EPWP) ratio regularly change with the increase of the ratio of number of cycles. The concept of EPWP increment ratio is introduced. Two new concepts of the effective dynamic shear stress ratio and the log decrement of effective stress are defined. It is found that the development of the EPWP increment ratio can be divided into three stages: descending stage, stable stage and ascending stage. At the stable and ascending stages, a satisfactory linear relationship is obtained between the accumulative EPWP increment ratio and the natural logarithm of the effective dynamic shear stress ratio. Accordingly it is deduced that the EPWP increment ratio at the number of cycle N is in direct proportion to the log decrement of the effective stress at the number of cycle N-1, but is independent of the cyclic stress amplitude. Based on the above analysis, a new EPWP increment model is developed, and it fits the test results for better prediction of the curves of EPWP generation.