The invention relates to synchronous grouting slurry for shield construction of a water-rich stratum and a preparation method thereof. The synchronous grouting slurry comprises a component A, a component B and a component C, wherein the raw materials of the component A comprise silicate 42.5-grade cement, II-grade fly ash, river sand and sodium bentonite, the raw materials of the component B comprise hydroxyethyl methyl cellulose and tap water, the raw material of the component C comprises nano silicon dioxide, the tap water, the ratio of the blending amount of the component B to the blending amount of the component A is 3: 5, and the ratio of the blending amount of the component C to the blending amount of the component A is 1: 2; the synchronous grouting slurry is high in early-stage shear strength, good in groutability, stable, high in stone rate, low in bleeding rate and good in dilution resistance, certain shear resistance is provided after the synchronous grouting slurry is injected, then duct pieces are inhibited from floating upwards, the effect of stabilizing the duct pieces can be achieved, and the problems of cracking and water seepage of the duct pieces are solved.