The sedimentation characteristics of the magnetic polishing liquid in the magnetorheological polishing area is an important part of the material removal mechanism in the polishing process. Aiming at the ultra-large-scale nonlinear problem of numerical calculation of polishing powder sedimentation characteristics, this paper solves the ultra-large-scale fluid-structure interaction calculation problem based on Kahan linearization. The sedimentation characteristics of the multiphase flow of polishing powder composed of hydroxy iron powder and silicone oil in the polishing area under gradient magnetic field were studied. Taking the polishing powder composed of hydroxyl iron powder with mass fraction of 70% and particle size of 5um and silicone oil with viscosity of 0.973 Pa·s as the research object, sedimentation analysis of different polishing wheel rotation speeds, different embedding depths and different mass fractions of carbonyl iron powder were realized. It is found that the polishing powder in the magnetorheological polishing area will increase with the increase of the rotating speed of the polishing wheel; when it reaches the outlet, the distribution of the polishing powder tends to be stable; the polishing powder will increase with the increase of the embedded depth and there is a saturation zone; the mass fraction of hydroxy iron powder affects the sedimentation ability in a non-linear manner.