Abstract: A new simplified effective stress constitut ive model is presented for evaluating small to large cyclic strains of saturated sand dur ing liquefaction process.It is developed by refining and modifying Ramberg-Osg ood model in the following three aspects:①A formulation for asymptotic movement s of both MPTL(Moving Phase Transformation Line)and MCSL(Moving Critical Stat e Line)is incorporated to accurately describe the effective stress path and the critical stress state;②A stress degradation model is used to describe the stres s-dilatancy and to define the degradation and hardening of the skeleton curve w ith variation of excess pore water pressure;③A new post-liquefaction deformation theo ry is incorporated to model large shear strain response in cyclic undrained loading after initial liquefaction.The results calculated by the proposed model have been compared favorably with experimental observations.The new model is s uitable for dynamic response analysis of level or nearly level saturated sand gr ound.