The invention relates to a preparation method for nano particles and nano-particle sol and application of the nano particles and the nano-particle sol in magnetic resonance imaging contrast medium. The preparation method of the nano-particle sol comprises the following steps: dissolving compound of ferric iron and compound containing divalent metal ions into aqueous solution of compound containing hydroxyl or carboxyl to carry out reaction, wherein the divalent metal ions are selected from Fe2+, Zn2+, Ni2+ or Mn2+; and reacting to obtain the sol containing MFe2O4 nano particles, wherein M is Fe, Zn, Ni or Mn. Compared with the prior art, the preparation method for the nano-particle sol has simple preparation method, low preparation cost and no need of nitrogen protection in the reaction process, the nano particle product has the advantages of small particle size, narrow particle size distribution, controllable size, good water solubility, good biocompatibility and the like.