The invention discloses a modified anti-cracking concrete expanding agent and a preparation method thereof. The expanding agent is prepared from the following raw materials in parts by weight: 10-20 parts of modified calcium oxide, 1-5 parts of calcium sulphoaluminate, 3-5 parts of fly ash, 10-20 parts of fluorgypsum, 5-15 parts of aluminum oxide, 5-15 parts of magnesite, 2-4 parts of calcium carbonate and 8-12 parts of modified expandable graphite. According to the invention, the calcium sulphoaluminate is modified through a composite modifier, then the calcium sulphoaluminate is compounded with the modified calcium oxide to prepare an expanding agent A, an expanding agent B is prepared from raw materials such as the magnesite, the fluorgypsum and the calcium carbonate, and finally, the expanding A, the expanding B and expandable graphite are ball-milled, screened, mixed and stirred to obtain the modified expanding agent, the expanding A, the expanding B and the expandable graphite have synergistic effect, and an expanding effect is achieved; and under the action of a stirrer, the stirring effect is improved, the production efficiency is also improved, a hardening phenomenon is avoided, the production cost is reduced, materials are uniformly mixed, and the product quality is improved.