Abstract: In real projects,soils under foundations or roadbeds are usually in complex initial stress state,deformation and settlement happens under earthquakes or other dynamic loadings. Conventional laboratory experiments of soils cannot authentically reproduce dynamic behaviors of soils under this kind of consolidation stress conditionsAfter having reformed the DTC199 torsional cyclic load triaxial apparatus,dynamic deformation of compacted loess can be tested when the initial principal stress axis is rotated. Results show that the initial angle  of the principal stress has an influence on the dynamic shear modulus of the compacted loess when other consolidation conditions are fixed. As  increases,the dynamic shear modulus shows a decreasing trend,and the maximum dynamic shear stress reduces gradually. But  does not considerably influence the maximum dynamic shear modulus. The initial angle  of the principal stress has basically no influence on the damping ratio of compacted loess. The damping ratio has an increasing trend with the increase of dynamic shear strain in the semilog plot of -lgd,and they show good relativity．