Abstract: The inductance gradient of the background field enhancement scheme of the four-rail electromagnetic launcher is simulated. Based on the principle of virtual work, the formula of the inductance gradient of the four-rail launchers under the background field is derived. A three-dimensional background field simulation model is established to analyze the variation law of inductance gradient under different main and additional rail parameters. The simulation results show that the inductance gradient of the system can be improved by increasing the launcher caliber, reducing the distance between the main and additional rails and the cross-sectional area of the additional rails. With the enhancement of background field, the proximity effect becomes obvious when the height of main rail reaches 57% of the caliber. Under the same cross-sectional area, the thickness of the additional rails should be reduced to increase the inductance gradient of the system, and the height of the additional rails should be reduced to alleviate the proximity effect. Concave cross-section additional rail can obviously improve the current proximity effect.