Abstract: β-hemihydrate gypsum was prepared through the synergistic modification of calcination and ball milling, and the effects of ball milling modification on its particle size distribution, specific surface area, and microstructure were analyzed in this study. On this basis, it was compounded with fly ash and slag to form an ultra-fine composite admixture (UCA) for concrete preparation. The mechanical properties and durability of the concrete were tested, and the enhancement effect and mechanism of ball-milled modified desulfurized gypsum on concrete performance were investigated. Results indicate that 12.17% of free water and adsorbed water in desulfurized gypsum are removed by the drying. A conversion rate of over 88.89% from desulfurized gypsum to β-hemihydrate gypsum is achieved when calcination is conducted at temperatures above 160 ^oC, and the crystallinity of the product meets the requirements for high-performance admixtures. After ball milling for 200 s and 400 s, the average particle size of β-hemihydrate gypsum is reduced from 81.22 μm to 54.14 μm and 50.75 μm, respectively, while its specific surface area is increased from 951 m²/kg to 1062 m²/kg and 1185 m²/kg. With the extension of ball milling duration, the 7 d compressive strength of concrete is increased by 27.78% and 35.93%, and the 28d compressive strength is increased by 38.33% and 41.63% by UCA-200 and UCA-400 (UCA prepared using β-hemihydrate gypsum ball-milled for 200 s and 400 s), respectively. Meanwhile, the freeze-thaw resistance and impermeability of concrete are optimized synchronously. The modification mechanism is as follows, the pore structure of concrete is optimized through the physical filling effect of ultra-fine particles. In the early stage, SO₄²⁻ is released from β-hemihydrate gypsum to react with C₃A in cement for ettringite formation. And in the later stage, the pozzolanic activity of mineral admixtures is stimulated to promote the secondary generation of C—S—H gel. These two effects synergistically improve the strength and durability of concrete. Volume stability risks can be avoided through measures such as optimization of the ultra-fine composite admixture content to 10%, providing technical support for the application of desulfurized gypsum in green building materials.