Astract: Traditional magnetorheological fluids are limited by the problem of dispersed phase particle sedimentation, so their application is greatly hindered. At present, the methods commonly used to improve the sedimentation stability of magnetorheological fluids are usually accompanied by a large loss of magnetorheological effect, which leads to the inability to give consideration to comprehensive properties. Therefore, how to strike a balance between magnetorheological effect and settlement stability is a huge challenge. Nano-iron particles with good magnetic properties are a good choice to achieve this balance. In this paper, nano-iron particles were prepared by DC arc plasma method, and nano-iron particles were used as dispersed phase to prepare magnetorheological fluids, and then nano-iron particles were doped with micron carbonyl iron powder to prepare nano-iron particles/micron carbonyl iron powder co-doped magnetorheological fluids with different doping ratios. The sedimentation stability and rheological properties of magnetorheological fluid were evaluated, and compared with micron-sized carbonyl iron powder magnetorheological fluid to explore the influence law and mechanism. The surface morphology, phase analysis and magnetic properties of nano-iron particles prepared by DC arc plasma method were analyzed The results show that it is spherical nanoparticles with high purity, high saturation magnetization (146 emu/g) and particle size of 30-50 nm. The rheological properties and sedimentation stability of nano-iron particle magnetorheological fluid prepared with nano-iron particles as dispersed phase were tested. The results showed that its shear yield strength was slightly lower than that of carbonyl iron powder magnetorheological fluid, but its sedimentation stability showed significant advantages compared with carbonyl iron powder magnetorheological fluid. The rheological properties and sedimentation stability of magnetorheological fluids co-doped with nano-iron particles and micron carbonyl iron powder with different doping ratios were evaluated, and the influence law and mechanism of doping ratio of micro-nano particles on the properties of magnetorheological fluids were analyzed. The results show that with the increase of the proportion of nano-iron particles, the sedimentation stability of nano-iron particles/micron carbonyl iron powder co-doped magnetorheological fluid is significantly improved, which is attributed to the joint help of nano-iron particles in free state and adsorption state in magnetorheological fluid. The influence of nano-iron particles on the shear yield strength of magnetorheological fluids is the result of the joint action of structural strengthening effect and magnetization weakening effect. In a certain range of magnetic field strength, compared with micron-sized carbonyl iron powder magnetorheological fluid, nano-iron particles/micron-sized carbonyl iron powder co-doped magnetorheological fluid with a certain doping ratio can improve the magnetorheological effect and settlement stability of magnetorheological fluid at the same time.