The invention provides a silt foundation material and a method for constructing a roadbed using the foundation material. The foundation material is composed of 50-70 parts of silt, 6-8 parts of silt curing agent, 6-12 parts of fly ash, 6-12 parts of slag powder, 6-12 parts of cement, and 0.8-1.5 parts of secondary curing agent, 3-4 parts of latex powder, 0.3-0.5 part of stabilizer; Described sludge curing agent is made of slag, modified diatomite, attapulgite, microsilica fume, powdered active carbon, peroxide, potassium permanganate, iron powder, polyrotaxane, sodium polyacrylate, alkaline activator, water reducer, thickener. The first choice is to solidify the silt with a silt curing agent, and then add other raw materials and use a secondary curing agent for secondary curing to prepare a silt foundation material. The sludge solidification process of the invention is simple, short in time and high in efficiency, and the solidified sludge will not cause secondary pollution, and can meet the requirements of roadbed materials.