Abstract: This paper aims to find out the interaction mechanism between the expansive component and the aggregate in the coarse-grained filler of the high-speed railway subgrade. We select some sections of the ballastless track of the Westem China Passenger Dedicated Line. These sections are deformed by arches. In some of the work sites, the packing of the bed part is collected for sampling analysis, and the expansion test with the porosity-montmorillonite content as a variable is carried out. The variation law of filler expansion rate under the combined action of montmorillonite content and filler porosity is analyzed for the expansion mechanism of this micro-expansion filler. The test results show that the filler expansion rate is positively correlated with the content of expansive minerals(montmorillonite) and negatively correlated with porosity. After further analysis of these data, the parameter β is proposed and defined as the ratio of montmorillonite content to filler porosity for the index of ductility expansion of the roadbed filler. When the bed group A of the structure β=0.365, the expansion property of the group A filler reaches the critical index of the high-speed railway. Conversion of parameter β to expansion rate and numerical simulation proves the incompatibility of the current regulations with the design calculation of the high-speed railway.