Dynamic characters of saturated soft clay strata have been studied under unconsolidated undrained condition based on the cyclic torsional shear test results. Stress equivalent failure relationship and strain equivalent failure relationship have been studied under different confining pressures and different stress combinations through cyclic torsional shear tests and cyclic triaxial tests. Analyzing the tests results of different test stress states

it is surprising finding that saturated soft clay followed Mises criterion under static combined cyclic loads as same as under static loads. And the cyclic strength is an invariable on certain static loads and certain number of circles to failure. Further

the pseudo-static elastoplastic cyclic creep relation

that is

the changing law of cyclic accumulate strain with static loads and cyclic loads as well as number of circles to failure is built through cyclic triaxial tests results. And this relation is in harmony with cyclic torsional shear test results. Then we can draw a conclusion that cyclic stress followed Mises criterion and cyclic failure course met pseudo-static elastoplastic cyclic creep for saturated soft clay under unconsolidated undrained condition. These conclusions not related to the testing stress states and confining pressures

it can be popularized in general stress states. It is the basis for research cyclic failure mechanism of saturated soft clay.