Magnetic Fe₃O₄ nanomaterials have excellent magnetic properties including high magnetic response, superparamagnetism and biocompatibility. By regulating the microscopic morphology of the materials, it is possible to achieve effectively control of its physical and chemical properties, such as energy storage, catalysis and medicine, in accordance with the requirements of practical applications. In this paper, according to modify the conditions including the ratio of Fe³⁺ and Fe²⁺ in the precursor, reaction temperature and reaction time, spherical magnetic Fe₃O₄ nanoparticles with particle sizes ranging from 10 to 25 nm were synthesized by a facile hydrothermal method without the addition of a morphology modifier. And the saturation magnetization intensity of the material under the optimum conditions was 73.3 emu·g^-1. The results show that the magnetic properties of Fe₃O₄ nanoparticles have a clear relationship with the preparation conditions. With the increase of hydrothermal temperature and time, the particle size and saturation magnetization intensity of magnetic nanoparticles increase first and then decrease. The smaller the particle size is, the smaller the internal magnetic effective volume is, and therefore the lower the saturation magnetization intensity is.