The invention discloses a concrete expansive agent, which belongs to the technical field of concrete additives. The concrete expansive agent comprises the following raw materials in parts by weight: 20-30 parts of coal ash, 5-15 parts of gypsum powder, 5-15 parts of alumstone, 2-6 parts of aluminum oxygen chamotte, 10-30 parts of calcium sulphoaluminate, 4-12 parts of aluminum slag, 1-5 parts of calcium carbonate powder, 4-12 parts of nano aluminum titanate fiber, 8-16 parts of rosin rock ore sand, 5-15 parts of vermiculite power, 2-6 parts of polypropylene fiber, 3-9 parts of cellulose acetate butyrate, and 3-12 parts of triethanolamine. The concrete expansive agent is high in expansion rate and low in alkalinity, is capable of improving the property of concrete, preventing the concrete from cracking, meanwhile reducing leakage and improving strength, and is low in price, simple in production process, low in cost, and suitable for large-scale production.