The invention relates to a polymine-coated ferroferric oxide magnetic nanoparticle, which is prepared by the method comprising the following steps: adding water-soluble trivalent ferric salt into ethylene glycol and then adding polymine and acetate, and putting the mixture into a sealed reaction kettle for solvothermal reaction for 5 to 60 hours at the temperature from 200 to 280 DEG C; separating out precipitates; and washing and drying the resultant precipitates with ethanol to result in black powdery polymine-coated ferroferric oxide magnetic nanoparticle. The method provided according to the invention has the advantages of: no need of severe equipment condition, safe, simple and convenient operation, simple raw materials, low production cost, good technological reproducibility, strongoperability, simple reaction condition, and being capable of achieve scaled-up production. The prepared Fe3O4 magnetic nano particle has the advantages of stable structural properties, adjustable particle diameter, narrow distribution of particle diameter, good magnetic property, great water solubility and biocompatibility and the like. The synthesized product can be extensively applied to the fields such as chemistry, biology, environment, medicine, etc.