The invention relates to a method for rapidly preparing high-viscosity mortar or concrete. The method comprises the following steps: (1) loading sand to be blended into a closed container, sealing and then vacuumizing; (2) sucking high-viscosity slurry into the vacuumized closed container through a slurry guide tube so that gaps among the sand are rapidly filled with the slurry under the vacuum condition by use of pressure difference, and the slurry wraps the surface of the sand well; and (3) after all the gaps among the sand are filled with the slurry, closing the slurry guide tube, and relieving vacuum so that the sand soaked with the slurry in the closed container becomes usable finished high-viscosity mortar. The high-viscosity mortar prepared by the method is good in quality; the method is efficient, speedy and convenient; and the manufacturing cost is low.