The invention discloses a modified concrete expansive agent, made from, by weight, 10-20 parts of calcium silicoaluminate, 25-30 parts of calcium sulfoaluminate, 20-25 parts of sulfur fixing waste residue, 10-20 parts of modified red clay, 1-5 parts of tourmaline powder, 6-8 parts of iron slag, 4-6 parts of modified calcium oxide, 4-6 parts of magnesium oxide, 3-5 parts of sodium methyl silicate, 2-4 parts of calcium formate, 0.1-0.3 part of azodicarbonamide, 0.4-0.7 part of maltodextrin, 1-4 parts of sodium lignin sulfonate, 0.3-0.6 part of polyglycol ether, 1-3 parts of a polycarboxylate superplasticizer, 2-5 parts of modified diatomite powder, and 0.5-0.8 part of nano calcium carbonate. The modified concrete expansive agent has the advantages of high strength, good compensating capacity and the ability to control calcium oxide reaction and is a promising concrete admixture.