The invention relates to a silt hardening agent. The silt hardening agent is characterized by comprising, by weight, 1%-5% of sodium polyacrylate, 2%-8% of biomass activated carbon, 2%-7% of sodium lignin sulfonate, 1%-5% of polyrotaxane, 4%-25% of reduced graphene oxide, 15%-50% of cement, 5%-25% of quicklime and 10%-35% of coal ash. The hardening agent is non-toxic and harmless and can be used on a large scale; synergistic effect among the raw materials is achieved, moisture content of silt can be effectively reduced, and strength of the silt is improved to meet requirements of earthwork reclamation; heavy metals and organic substances in the silt can be effectively sealed, and risk of secondary exposure of pollutants is avoided.