In order to enhance the theory and process of magnetorheological polishing fluid (MRPF) preparation, the bubble models of MRPF under ultrasonic preparation and mechanical preparation were established by considering the mixture continuity equation and the VAND viscosity equation. The bubble motion of MRPF was investigated numerically by using the fourth-order Runge-Kutta algorithm, and the influence of liquid phase and liquid-solid two-phase on the bubble of MRPF was discussed. The results show that the bubble in MRPF undergoes the dynamic processes of growth, expansion, compression, collapse, and rebound under ultrasonic preparation, while it only undergoes the dynamic processes of compression and rebound under mechanical preparation. For the ultrasonic preparation process, the addition of solid particles of MRPF significantly weakens the original cavitation effect of liquid phase, while for the mechanical preparation process, the addition of solid particles of MRPF enhances the original cavitation effect of liquid phase. The pressure inside bubble and the wall velocity of bubble produced by ultrasonic preparation of MRPF were 104 times higher than those of the mechanical preparation. The analysis results of the bubble dynamics of MRPF fit the experimental results of sedimentation rate of MRPF well. The superiority of ultrasonic preparation of MRPF was theoretically verified, providing a theoretical basis for further insight into the homogenization mechanism of MRPF.