The dynamic torsional shear triaxial tests are carried out on undisturbed loess of Wugong (Shaanxi province) under different water contents and different initial average principal stresses conditions; and the changing laws of dynamic shear modulus and damping ratio of undisturbed loess under pre-shearing stresses are obtained. Results show that the general trend of the changing law of dynamic shear modulus of undisturbed loess does not change with the changing of the dynamic shear strains under the condition of pre-shearing stresses. The degrees of effects of water content w and initial average principal stress pm0 on the dynamic shear modulus of undisturbed loess reduce under the condition of pre-shearing stresses. Water content w and initial average principal stress pm0 have significant influences on the initial dynamic shear modulus of undisturbed loess under the condition of pre-shearing stresses. Damping ratio λ of undisturbed loess under the condition of pre-shearing stress first decreases with the increase of the dynamic shear strain

and then increases gradually. In general

the values of the damping ratios are between 0.2 and 0.35. The existing method of using semilogarithmic coordinates to fit the line equation is no longer applicable to the calculation of the damping ratios of undisturbed loess under the condition of pre-shearing stresses. The effects of w on λ–γd relations of undisturbed loess show a trend that the higher w

the larger λ. The laws of effects of pm0 on λ–γd relations of loess are not obvious