Abstract: In this study, PAO3 based magnetic fluids were prepared from oleic acid modified nano Fe₃O₄, polyisobutylene amine (PIBA) and polyolefin (PAO3), and PAO3 oil was used as the control sample. XRD, TEM and VEM showed that the synthesized Fe₃O₄ nanoparticles had good crystallinity, spherical shape, 10 nm diameter and good paramagnetism. The sedimentation experiment showed that the prepared magnetic fluid had good dispersion stability and meets the standard of magnetic fluid. The tribological properties of PAO3 oil on 12CrNi4A steel /304 steel, 12CrNi4A steel /GCr15 steel and 12CrNi4A steel /Si₃N₄ ceramic friction pairs were tested with PAO3 oil as the control group. It was found that the friction coefficient and wear loss always decreased when 304 steel was used as the friction pair; When GCr15 steel was used as a friction pair, the friction coefficient and wear amount always increased; When Si₃N₄ ceramic ball was used as the upper friction pair, the friction coefficient of the friction pair decreased and the wear amount increased. This meant that the nano Fe₃O₄ particles in the magnetic fluid had different effects on the surface under different wear degrees. Therefore, the changes of wear trace morphology and wear trace elements of 12CrNi4A steel /GCr15 steel and 12CrNi4A steel /Si₃N₄ ceramic friction pairs under different grinding time were observed by SEM and EDS. The results showed that the effect of nano Fe₃O₄ particles on the friction surface was a process of first destroying the oxide film and then repairing the oxide film. When the oxide film on the surface can lubricate the surface of the friction pair, the nano Fe₃O₄ particles accelerated the destruction of the oxide film on the surface; When the surface oxide film was destroyed, nano Fe₃O₄ particles promoted the formation of surface oxide film. Therefore, the existing state of oxide film on the surface of 12CrNi4A steel /GCr15 steel was changed through the change of load. The results showed that when the load increased to 150 N, the removal rate of oxide film on the surface of GCr15 steel was faster than the reformation rate. At this time, the friction and wear can be greatly reduced by using magnetic fluid lubrication. This proved that the influence of nano Fe₃O₄ particles on the friction surface was a process of destroying the oxide film first and then repairing the oxide film. In the12CrNi4A steel /Si₃N₄ ceramic friction pair, the reason why the friction coefficient decreased and the wear amount increases was that the protective effect of nano Fe₃O₄ particles on the friction surface had not offset its damage to the surface. Therefore, when the friction time was prolonged to 1 h, the results also proved that the effect of nano Fe₃O₄ particles on the friction surface was a process of destroying the oxide film first and then repairing the oxide film. During the experiment, it was found that the concentration of magnetic fluid also had an impact on the tribological properties. Therefore, the tribological properties of magnetic fluid with different concentrations were studied. It was found that the most concentration existed in the three pairs, which minimized the friction coefficient and wear amount. Finally, the effect of magnetic field on the tribological properties of magnetic fluid was investigated. It was found that there was a synergistic effect between the applied magnetic field and the concentration. Compared with the magnetic fluid with the lowest concentration, the applied magnetic field could improve its tribological properties. On the contrary, when the concentration of magnetic fluid was higher than the optimal concentration, the friction and wear would be aggravated. In general, magnetic fluid is sensitive to the surface of friction pair, and friction and wear can be reduced only under appropriate conditions. And the tribological performance can be optimized through the synergistic effect of concentration and magnetic field. This work provided guidance for the application of magnetic fluid in the lubrication of aviation friction parts.