Abstract: Through the in-situ dynamic load experiments on the test section along a certain high speed railway,the dynamic stress,elasto plastic deformation and compositive dynamic stiffness at different sections and depths are obtained.The engineering characteristics and dynamic responses of the bridge-subgrade transition section are studied under the simulated train dynamic loads.It is verified that ① the attenuation effect of the dynamic stress in the subgrade composed of graded broken stones is very obvious.The attenuation coefficient of the dynamic stress on the subgade surface layer decreases gradually and the subgrade bottom is contrary.② The elasto-plastic deformation concentrated on the surface layer of the subgrade increases gradually along the longitudinal direction and contrary along the depth direction.③ The compositive dynamic stiffness of the subgrade relates with the stiffnesses of the surface layer,the bottom layer and the embankment.In a word,the subgrade of bridge-subgrade transition section is of the transition function from the transfer laws of the dynamic stress,elasto plastic deformation and compositive dynamic stiffness of the subgrade.