In the conditions of ultrasonic auxiliary and no inert gas protection, Fe3O4 nanoparticles were prepared successfully by the combination of oxidation-coprecipitation method and chemical coprecipitation method, and were characterized by XRD, FT-IR and SEM. The results show that the Fe3O4 nanoparticles have a better crystal structure, and the particle size is about 7nm. Orthogonal experiments prove that ultrasonic time is the main factor that affects the particle size of Fe3O4 nanoparticles, and the molar ratio of Fe2 to Fe3 is the secondary factor. Water-based magnetic fluid was prepared through the surface modification of Fe3O4 nanoparticles by citric acid. The magnetization curve indicates that its coercivity and remanent magnetization are low, and shows good superparamagnetism.