The invention provides a modified calcium oxide expansive agent used for cement concrete and a preparation method thereof. The modified expansive agent used for cement concrete is prepared by bonding a silane coupling agent with an acrylic polymer and a calcium oxide expanding clinker, wherein the silane coupling agent accounts for 0.1-10% of the modified expansive agent used for cement concrete by mass; the acrylic polymer accounts for 0.1-10% of the modified expansive agent used for cement concrete by mass. The modified calcium oxide expansive agent used for cement concrete has the beneficial effects that invalid hydration at the plastic stage of concrete can be reduced, the expansion quantity after hardening is increased, the shelf life of the expansive agent product is prolonged, the crack and seepage resistance of a concrete structure is improved and the durability of concrete is substantially improved.