Unlike in the emulsion preparation method，magnetic Fe3O4-chitosan（CS) nanoparticles were prepared by cross-linking with sodium tripolyphosphate（TPP),precipitation with NaOH and oxidation with O2 in hydrochloric acid aqueous phase containing chitosan and Fe(OH)2. The effects of the sequence of addition of NaOH and TPP，the reaction temperature，and the ratio of CS/Fe(OH)2 were studied. The magnetic composite nanoparticles were characterised by TEM，X-Ray and magnetic intensity. Adding NaOH after TPP was better，and the most suitable reaction temperature was 60℃. TEM showed that the diameter of composite nanoparticles was about 80 nm，and that the magnetic Fe3O4 nanoparticles with a diameter of 20 nm were evenly dispersed in the chitosan materials. Magnetic measurement revealed that the saturated magnetisation of the Fe3O4-Chitosan nanoparticles reached 35.54 emu/g when mFe/mCS=0.672 and that the nanoparticles showed the characteristics of superparamagnetism. X-ray diffraction patterns(XRD)indicated that the coating of CS did not result in a phase change in Fe3O4. Thermogravimetric analysis indicated that almost all the Fe2+ in the aqueous solution was entrapped in the chitosan nanoparticles.