The invention discloses a method for preparing high magnetic resonance sensitivity ferroferric oxide nano-particle with tumor-targeting function. Amphiphilic block polymer of the end capping of tumor-targeted ligand and super-paramagnetic ferroferric oxide are dissolved in tetrahydrofuran, then the obtained solution is added into ionized water or distilled water, the tetrahydrofuran is volatilized, the super-paramagnetic ferroferric in the solution is separated out, washed by the distilled water, then dispersed into the ionized water or the distilled water, and finally centrifugalized, thus the nano-particle is obtained; the tumor-targeted ligand is folic acid, polypeptide or antibody. In the invention, by loading lipophilic ferroferric oxide nano-particle at self-assembled micellar inner core, the tumor-targeted and water-soluble ferroferric oxide nano-particle that is below 50 nm is developed. The tumor-targeting function is realized by coupling the folic acid or antibody at micellar surface. The invention has wide application prospect.