Abstract: Dynamic deformation behavior tests on dam materials are conducted using a large scale triaxial test apparatus.The effect of confining pressure and consolidated stress ratio on the maximum dynamic modulus,reduction of dynamic modulus and damping ratio is analyzed.It is shown that the maximum dynamic modulus increases with the increase of the confining pressure and consolidated stress ratio,and that the dynamic modulus decreases with the increase of the dynamic strain.The consolidated stress ratio has little effect on the maximum damping ratio.The modified formula of the maximum dynamic modulus and reduction of dynamic modulus with normalized dynamic strain is developed on the basis of the test results.This formula can describe the effect of the confining pressure and consolidated stress ratio on the dynamic modulus.The effect on dynamic deformation of reduced scale is discussed using particles finer than 5 mm.Compared with the field soil,the maximum dynamic modulus of the soil from laboratory tests is smaller,and the maximum damping ratio of the soil from laboratory tests is larger.