Abstract：By collecting loess samples in typical slope along Zhengzhou—Xian passenger special line，the dynamic shear strength and dynamic stress-dynamic strain relationship of Q3 loess acquired by loess undrained dynamic triaxial test under cyclic load are discussed. The results show that with the increase of cycle number，the strain of Q3 loess develops. When the dynamic load is less than preliminary consolidation pressure，the dynamic strain increases in several initial cycles，and strain rate decreases after cycle number with that strain curve becomes flat；once exceeding preliminary consolidation pressure，the strain increases rapidly and eventually leads to soil damage. Dynamic elastic modulus with increasing strain gradually decreases，and this relationship can be described as hyperbolic form. The dynamic shear strength lower than static shear strength decreases with cycle number. Based on these results，the loess slope dynamic model is established，and the slope deformation rules are studied. The rules show that the instability process is an unstable one as stress，displacement，velocity and acceleration of slope change with time. The lower slope has great displacement and becomes instable more easily than the corresponding upper one. Subjected to train vibration load，the dynamic shear strength of loess decreases，which leads loess slope stability to decrease. Compared with the upper slope，the lower slope is apt to be instable easily.
