Abstract: The limit equilibrium condition and liquefaction failure process of saturated sandy soils under cyclic loading are discussed in this paper. As a result, an extended Coulomb's equation for analyzing the seismic stability of saturated sand layer is proposed, In accordance with the data of sand liquefaction test, an approach is advanced to determine the dynamic shear strength in the form of effective stress in saturated sandy soils. For practical engineering purposes, the liquefaction failure process of saturated sandy soils under cyclic loading is separated into two phases and the turns of these two phases can be regarded as the failure criteria of the sandy soils. In addition, the generalized relationships between cyclic shear stress on failure plane and the number of cyclic loading causing liquefaction of the sandy soils are also presented.