Abstract: A rare earth-based nanoparticle magnetic resonance contrast agent and a preparation method thereof are provided. The rare earth-based nanoparticle magnetic resonance contrast agent is rare earth-based inorganic nanoparticles having the surfaces coated with hydrophilic ligands. The rare earth-based nanoparticles are first obtained by a high-temperature oil phase reaction, and then the surfaces thereof are coated with hydrophilic molecules to obtain the rare earth-based nanoparticle magnetic resonance contrast agent. Compared with the existing clinical contrast agent, the magnetic resonance contrast agent of the present invention has a greatly improved relaxivity, a good imaging effect, a low required injection dose, and long in vivo residence time. In addition, the rigid structure of the inorganic nanoparticles can effectively reduce the leakage possibility of gadolinium ions.