The invention provides a modified calcium oxide expansive agent used for cement concrete and a preparation method thereof. The modified calcium oxide expansive agent used for cement concrete comprises polyvinyl alcohol and a calcium oxide expanding clinker, wherein the surface of the calcium oxide expanding clinker is coated with polyvinyl alcohol. The modified calcium oxide expansive agent used for cement concrete has the beneficial effects that the hydration process of the calcium oxide expanding clinker can be regulated; invalid hydration at the plastic stage of concrete is reduced, the expansion quantity after hardening is increased and the shelf life of the product is prolonged; the modified calcium oxide expansive agent used for cement concrete can be used for crack and seepage resistance of a concrete structure and can substantially improve the durability of concrete.