The invention relates to a modified calcium oxide expanding clinker as well as a preparation method and application thereof. The preparation method of the modified calcium oxide expanding clinker comprises the following steps: forming plenty of dense calcium oxalate on the surface of calcium oxide expanding clinker powder in an oxalic acid solvent through a physical absorption and chemical reaction, thus preparing the modified calcium oxide expanding clinker. The modified calcium oxide expanding clinker can be mixed with a dispersion supporter, thus forming a modified calcium oxide concrete expansive agent. The oxalated packing modified calcium oxide expanding clinker prepared by the preparation method and the expansive agent thereof have the beneficial effects that the expansion quantity and expansion stress of the expanding material in cement concrete are obviously increased, thus optimizing the expansion processes of the expanding clinker and the expansive agent thereof; the weather resistance and moisture resistance of the calcium oxide expanding clinker and the expansive agent thereof are improved.