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A novel closed-double-magnetic circuit (CDMC) was presented to achieve high and uniform magnetic flux density (MFD) in the long air-gap (LAG) of a long-stroke horizontal electromagnetic vibration exciter. First, the normal single-magnetic circuit (SMC) and the proposed CDMC were equivalent to the lumped parameter models based on the circuit equivalent principle, which were analyzed theoretically by the Kirchhoffs law and the superposition theorem. The comparison between the two circuits shows that the CDMC can realize more intensive and more uniform MFD in the LAG. To strengthen the uniformity of the MFD in the LAG, the improved CDMC with uneven air-gap was proposed. Then the MFD in the LAG was simulated and optimized with the finite-element method based on the prototype of a one-meter-stroke horizontal electromagnetic vibration exciter, and the influence of slits in the outer magnetic yoke for practical application was analyzed. The simulation matches the theoretical analysis quite well, which also stresses that the CDMC has less flux leakage than the SMC and the influence of the slits can be negligible. Finally, the experiment on the prototype also verifies the effectiveness of the proposed CDMC with the optimal air-gap.