The alluvial filling soil on the side of bridge piers and its physical,mechanical,and geometric pa-rameters have important effects on train vibration propagation. Taking a pier and its adjacent alluvial filling soil and foundation site along the Tieling-Siping section of the Harbin-Dalian high-speed railway as the research object in this study, a finite element model was established, and the "effective value of vibration change" was defined. Combined with the theory of gray relational degree, the field measurement and numerical simulation method were used to reveal the influence of the alluvial filling soil and its parameters on vibration propagation. The following conclusions are obtained through the analysis. (1) Vibration characteristics of the supporting pad stone at the top of the pier are different from those of alluvial filling soil and the foundation site, which have amplification effects on the train vibration. (2) The vibration acceleration level of the alluvial filling side is larger than that of the unfilled side, and a rebound phenomenon will be observed at the boundary position of the alluvial filling soil. (3) The vibration acceleration level increases with the elastic modulus of alluvial filling soil, and its density and Poisson's ratio hardly affect the vibration propagation. Within 1 m from the pier, the vibration acceleration level increases with the damping ratio; when the distance is larger than 1 m, this level decreases with the increase in the damping ratio. (4) The vibration acceleration level increases with the length of the alluvial filling soil, and its height demonstrates an optimum value, which minimizes the vibration acceleration level on the side of the alluvial filling soil. Meanwhile, the sensitivity of each parameter of alluvial filling soil on the pier side to the ground vibration is in this order: elastic modulus ＞ length ＞ damping ratio ＞ height ＞ density ＞ Poisson's ratio.