The invention discloses an improved roadbed filling material based on low liquid limit silt and a manufacturing method thereof, and belongs to the technical field of roadbed filling materials, wherein the improved roadbed filling materials comprise the following ingredients in the following proportion through being metered according to the total weight of the substrate filling materials: 2 to 4 percent of lime, 2 to 4 percent of fly ash and the balance silt. The lime and fly ash mixture is used for improving the low liquid limit silt; the permeability and the compaction performance of the low liquid limit silt can be reduced; the water stability of the roadbed filling soil is improved; in addition, the soil body intensity can be improved when the roadbed deformation is reduced; the improved silt can be used for highway roadbed filling materials; the building cost of a highway can be reduced.