Water-based Fe3O4 nano-magnetic fluid for seal was prepared by coprecipitation. In order to obtain nano-magnetic fluid preparation technology which can produce industrially, the choice of second coating surfactant has been studied. Factors affecting nano-particle size and particle saturation magnetization were investigated using five factors and three level orthogonal experimental design. The size of nano-particles and particle saturation magnetization were characterized by transmission electron microscopy (TEM) and WSM vibration magnetometer. The results showed that the adding speed of NaOH solution and heat-maintaining time after adding, adding speed and reaction temperature of sodium oleate solution, as well as the heating temperature of sodium oleate solution influence the properties of nano-magnetic particles more. With the optimum process parameters, Fe3O4 nano-magnetic fluid which is below 15nm and can meet the nano-magnetic fluid sealing technology requirements has been prepared.