Electromagnetic coils and permanent magnets were used to independently design a magnetorheological grease (MRG) shear test device with reverse demagnetization to jointly control the magnetic field. ANSYS Electronics was employed to simulate and analyze the magnetic field distribution of the test device, and MRG with different mass fractions of carbonyl iron powder was prepared. The shaking table test was performed to evaluate the dynamic shear properties of MRG under reverse demagnetization. The effects of field current, excitation amplitude, and excitation frequency on the dynamic performance of MRG were also investigated. Results show that the mass fraction of carbonyl iron powder has a considerable impact on the dynamic performance of MRG. With the increase in the field current, the shear stress of MRG rapidly increases and then slightly subsides, and the damping force decreases. The energy dissipation capacity of MRG increases with the displacement amplitude, while the damping force slightly increases with the loading frequency.