Abstract: Based on the study of cyclic triaxial and torsioaal shear tests of saturated sand, the transient effective shear strength theory of soil’ under cyclic loading is presented, which indicates that the transient effective shear strength of soil includes cohesion ct (saturated sand ct = 0), internal frictlonal resistance and dynamic viscousresistance and a general transient effective shear strength formula is founded. It is also verified by test results that the dynamic effective internal frictlonal angle and the dynamic viscosity coefficientη are stable soil behavior coefficients, and there is a stable strength failure plane in the effective frictional stress space which is comparable with Mastuoka’s Criterion, and thence the failure criterion of saturated sand under cyclic loading is given.