The invention discloses a lithionite slag admixture and its preparation method and use. The lithionite slag admixture is prepared from lithionite slag and diatomite. The preparation method of the lithionite slag admixture comprises drying at a low temperature of 105 DEG C, drying at a high temperature of 300 DEG C and grinding. The mixture of a lot of lithionite slag and concrete is used as a concrete admixture replacing cement and can be used for preparation of C30-C60 eco-friendly high-performance concrete products with different strength. The high-performance concrete product has later compressive strength greatly larger than that of common concrete and has the largest later compressive strength increasing rate of 46.64%. The lithionite slag admixture has substantial acid rain corrosion resistance, has a mass loss late of 51.3% and 34.3% that of the common concrete, has a low concrete compressive strength loss late, has the average compressive strength 1.23 times that of the common concrete, has obvious later acid rain resistance and has neutralization depth of 57.5% that of the common concrete. The lithionite slag concrete has high carbonization resistance and splitting tensile strength and low chloride ion permeability.