Ferroferric oxide（Fe3O4） magnetic nanoparticles are widely used as passive targeting carriers in gene therapy, due to their simple preparation, targeting under external magnetic field and easy surface grafting. This study synthesized oil phase Fe3O4 nanoparticles with controllable particle sizes in the range from 4 to 9 nm by regulating the accumulation growth time in the solvothermal method. Then, meso-2, 3-dimercaptosuccinic（DMSA）was employed to double exchange oleic acid molecules on its surface to provide good water dispersibility. Finally,Fe3O4-DMSA-PEI magnetic nanoparticles were obtained by grafting branched polyethylenimine（PEI） onto Fe3O4-DMSA surface through amidization reaction. The results demonstrate that the Fe3O4-DMSA-PEI magnetic nanoparticles have a surface Zeta potential of（52.50 ± 1.94） mV, remaining a certain degree of superparamagnetism（14.48 emu/g, 1 emu/g=1 A·m2/kg）. When the mass ratio of Fe3O4-DMSA-PEI magnetic nanoparticles to plasmid DNA is 15 : 1, it can completely block DNA and it