Abstract: Ultrafine gypsum powder was prepared with sodium hexametaphosphate as a grinding agent by colloid mill, and ultra-fine grinding products were characterized through methods such as laser particle size analysis and XRD. The results showed that when the slurry concentration was 20%, and the discharge width was 12 mm, and the dosage of sodium hexametaphosphate was 0.20%, and the ultrafine grinding time was 15 min, the superfine gypsum powder with d₅₀ of 11.12 μm could be obtained. Moreover, the change of grain size and micro-strain of gypsum particles caused by ultra-fine grinding damage the lattice, resulting in the decrease of crystallinity ultrafine grinding products and the tendency of amorphous. In addition, hydrogen phosphate ions could be adsorbed on the surface of gypsum particles to make the ζ potential of gypsum surface shifted from -8.83 mV to -13.39 mV, and the turbidity of gypsum slurry increased from 13 NTU to 17 NTU while its viscosity decreased from 72.49 mPa·s to 48.91 mPa·s, which promotes the dispersion of gypsum particles. Under the combined action of the above factors, the particle size of superfine gypsum powder was reduced and the superfine grinding efficiency of gypsum was improved. The research has certain reference significance for preparing ultrafine gypsum powder and improving the quality of ultrafine gypsum powder.