Orthogonal tests in two rotational modes (the rotational tool mode and the rotational worktable mode) are conducted to confirm the synergistic effect mechanism of an electro-magnetically coupled field in the Electro-Magneto-Rheological(EMR) effect-based tiny-grinding wheel finishing. Under the electro-magnetically coupled field, the EMR polarized particles of the rotational tiny-grinding wheel generate the Lorentz force and the spin couple, then the in situ vibration of the EMR particles due to the spin couple can impact regularly on the workpiece surface and promote material removal, so the material removal depth in the rotational tool mode is larger than that in the rotational worktable mode. However, if the interaction force of the particle chains is weak, the in situ vibration will break the particle chains and reduce the material removal. The couple mode of electric and magnetic field has a significant influence on the machining efficiency of the EMR finishing. It shows a good synergistic effect of the EMR finishing in the voltage of electric field of 1 kV and the excitation voltage of magnetic field of 5 V in the rotational tool mode.