The invention belongs to the technical field of building materials and particularly relates to a high-activity mineral admixture and a preparation method thereof. limestone, granulated blast furnace ferro-nickel slag and steel slag are mixed in proportion, the obtained mixture is added to a ball mill to e ground for 60-90 min, the grinding efficiency of the granulated blast furnace ferro-nickel slag can be effectively improved, the grinding power consumption can be reduced, then, desulfurized ash and an adhesion agent are mixed with the granulated blast furnace ferro-nickel slag and homogenized for 5-20 min, and the activity of the mineral admixture can be effectively improved by 5%-15%. The mineral admixture adopts wide materials, is low in price, simple to prepare and easy to popularize and apply and has significant economic and social benefits.