In this paper, a multiphase particle flow model of the transient process of magnetorheological finishing (MRF) has been proposed. The mechanical relations of the transient processes are obtained at macroscopic, mesoscopic, and microscopic levels. A transient process characterization method based on three-direction in situ mechanical signals has also been proposed. The three-direction mechanical signals of the MRF are synchronously and dynamically measured in situ. The problem of in situ measurements of the liquid-solid interface of the MRF in millimeter-scale space is solved. The high removal rate finishing fluid with a yield strength of 220 kPa and a Bingham viscosity of 0.07 Pa·s is used to conduct the MRF experiment on a BK7 ultra-precision planar element of ?50-mm, and the transient response time of the MRF flow is identified to be 700 ms.