Abstract: The static and dynamic universal triaxial and torsional shear apparatus of soil recently equipped in Dalian University of Technology is used to perform five types of tests for loose sands subjected to triaxial and torsional coupling cyclic shear and simple torsional shear. The effect of variation of principle stress orientation under cyclic loading on undrained cyclic shear behavior of Fujian Standard loose sand under isotropic consolidation condition is investigated. It has been shown through comparative tests that the change of orientation of principal stress during cyclic loading has a considerable influence on undrained dynamic strength of saturated loose sands. The dynamic strength under the cyclic rotation of principal stress axes is lowest among those obtained with five given types of stress paths of cyclic loading. The relationship between excess pore water pressure induced during vibration normalized with the initial effective confining pressure and the ratio of number of cycles with the cyclic number required at failure is independent of the variation mode of direction of principal stress during cyclic loading. Furthermore, both the change of the normalized excess pore water pressure with generalized shear strain and the change of the generalized shear strain with ratio of number of cycles are irrelevant to the change of orientation of principal stresses.