The attenuation of dynamic modulus of coarse-grained soil filling with the increase of the strain should be considered in the refined design of the high-speed railway subgrade. On the basis of a large number of past investigations by other researchers on dynamic modulus of coarse-grained soil at home and abroad, this paper analyzes the mechanical properties of coarse-grained soil filling in high-speed railway subgrade under cyclic train load, with characteristics of small confining pressure, high density, low water content and weak structural property. By summarizing the empirical models of dynamic modulus and the range of attenuation curves, a method to determine the value of dynamic modulus of coarse-grained soil filling in high-speed railway subgrade has been proposed, with the reasonable range of the working dynamic modulus ratio being discussed. According to the measured dynamic deformation of ballastless track subgrade for a number of high-speed railway lines in China, the average working dynamic modulus of graded gravel in the surface layer and class A&B filling material in the bottom layer of the subgrade are 0.60–0.80 times and 0.73–0.90 times of the maximum modulus, respectively.