The soils under the foundation of building or within the slope are usually in complex initial stress state before subjected to cyclic loading induced by earthquake shaking or wave loading. The conventional laboratory experiments of soils cannot authentically reproduce dynamic behavior of soils under complex consolidation stress states. The soil static and dynamic universal triaxial and torsional shear apparatus recently developed in Dalian University of Technology is employed to perform experimental tests on Fujian standard sand with relative density of Dr=30 % under complex initial stress state subjected to cyclic torsional shear loading. Through a series of soil tests

the effects of both initial orientation of principal stress and the initial ratio of deviatoric stress on dynamic modulus and damping ratio of sand are investigated systematically.