The invention discloses a superparamagnetic water-soluble manganous ferrite nano-particle which is prepared by a high temperature liquid phase pyrolysis method. Dendritic polymer taking benzene ring as core is taken as template agent, polyatomic alcohol can be used as organic solvent and surface active agent, organic iron compound and organic manganese compound are dissolved into the polyatomic alcohol solution of the template agent, and gradient heating and staged reaction can be carried out, so that the superparamagnetic water-soluble manganous ferrite nano-particle which has the average particle size of 5-8nm, good dispersibility and better spinel structure. The invention has the advantages of simple preparation method, easily obtained raw materials, low cost, mild reaction condition, low requirement for equipment, environment-friendly production process without by-product and public nuisance and large-scale industrialized production.