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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 modeled by lumped element circuit using equivalent circuit principle, analyzed theoretically by the Kirchhoff's law and the superposition theorem. The comparison between the two circuits shows that the CDMC can have 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 and its design method were proposed theoretically. Thereafter, the uneven air gap structure expressed as a three-line-segment form was presented and its optimization was also conducted based on a finite element model referring to a prototype of a one-meter-stroke horizontal electromagnetic vibration excite. Then, the magnetic flux leakage of the magnetic circuits and the influence of slits in the outer magnetic yoke for practical application was also analyzed with the finite element method (FEM). The simulation demonstrates that the CDMC with optimal air gap can further improve the uniformity of the MFD in the LAG. In addition, it is also indicated that the CDMC has less flux leakage than the SMC and the influence of slits can be negligible. Finally, the experiment on the prototype also verifies the effectiveness of proposed CDMC with optimal air gap.