The invention relates to a ferromagnetic nano material. An inner core is a ferroferric oxide nano cube; an intermediate layer is modified by adopting polycaprolactone and oleic acid; an outer layer is used for modifying a cationic polymer through an electrostatic attraction effect; the intermediate layer comprises two layers of the oleic acid and the polycaprolactone for connecting hydrophobic ends of the two layers of the oleic acid. The invention also relates to preparation and application of the ferromagnetic nano material. The ferromagnetic nano material can be stably dispersed in a water-phase system, and is used for realizing the high-efficiency marking and the MRI (Magnetic Resonance Imaging) T2 enhanced imaging of a stem cell.