In this paper, the Fe3O4 magnetic nanoparticles were prepared by coprecipitation method with sodium oleate as surfactant modification, and magnetic fluid was prepared with silicon oil as the carrier. The influence of different adding ammonia orders and different adding speed on the property of magnetic fluid was studied. The Fe3O4 magnetic fluid was tested by VSM, the settlement after standing and TEM. The reslut showed that when the ammonia added before the sodium oleate, the dispersivity of the magnetic liquid is the best and the saturation magnetization is 11.4emu/g. Furthermore, with dripping ammonia in a speed of 10ml/min before adding the sodium oleate, the dispersivity of the magnetic liquid is better and the saturation magnetization is 14.3emu/g.