The invention provides a modified calcium oxide cement concrete expansion agent and a preparation method thereof. The modified calcium oxide cement concrete expansion agent consists of acrylic resin and a calcium oxide expansion clinker, wherein the surface of the calcium oxide expansion clinker is wrapped with the acrylic resin; the mass of the acrylic resin accounts for 0.01 to 10 percent of the total mass of the modified calcium oxide cement concrete expansion agent; the specific surface area of the modified calcium oxide cement concrete expansion agent is 100 to 600m<2>/kg. According to the modified calcium oxide cement concrete expansion agent, a hydration process of the calcium oxide expansion clinker can be regulated, ineffective hydration in the plastic stage of concrete can be reduced, expansion after hardening can be increased, and the shelf life of the expansion agent product can be prolonged; the modified calcium oxide cement concrete expansion agent can be used for the crack resistance and seepage resistance of a concrete structure, and the durability of the concrete can be greatly improved.