Abstract: The nano Fe3O4 particles were prepared with chemical precipitation method. And then used sodium oleate and sodium dodecyl benzene sulfonate to double-coat, stabilized water based magnetic fluid was obtained. The crystallization state, surface functional groups, particle size and magnetic properties of nano-Fe3O4 particles were investigated with XRD, IR, TEM and VSM. Then the factors influenced the magnetic fluid preparation were discussed. The results show that the stability of the double-coated water based nano-Fe3O4 particles magnetic fluid was improved obviously and the saturation magnetization intensity was 47.06 Am2·kg-1.