The invention discloses a preparation method for a polysaccharide-modified water-soluble rare-earth up-conversion nano-material, belonging to the field of nano-materials. The preparation method comprises the following steps: synthesizing rare-earth up-conversion nanoparticles with sodium yttrium tetrafluoride as a matrix by using a solvothermal method; and then treating UCNPs with pullulan and allowing UCNPs to undergo an esterification reaction with oleic acid molecules so as to obtain the pullulan-modified water-soluble rare-earth up-conversion nano-material. The water-soluble NaY(Mn)F4: Yb, Er rare-earth up-conversion nano-material doped by Mn<2+> with a mol ratio of 30% has good water solubility, the red monochromatic band of the nano-material has high luminescence strength, the surface of the nano-material contains ester functional groups responsive to pH values, and pullulan can be used as a liver cancer-targeting carrier material; thus, the nano-material has potential application prospects in fluorescent labeling, biological imaging, photo-thermal treatment, drug carriers and the like.