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To improve the low-frequency vibration reduction effect of a steel spring floating slab track (FST), nonlinear quasi- zero-stiffness (QZS) vibration isolators composed of positive stiffness elements (PSEs) and negative stiffness elements (NSEs) were used to support the FST. First, considering the mechanical characteristics of the nonlinear QZS vibration isolators and the dynamic displacement limit (3 mm) of the FST, the feasible parameter groups were studied with the nonlinear stiffness variation range and bearing capacity as evaluation indices. A vertical vehicle?quasi-zero-stiffness floating slab track (QZS-FST) coupled dynamic model was then established. To obtain a reasonable nonlinear stiffness within a few millimeters, the original length of the NSEs must be analyzed first, because it chiefly determines the stiffness nonlinearity level. The compression length of the NSEs at the equilibrium position must be determined to obtain the low stiffness of the floating slab without vehicle load. Meanwhile, to meet the dynamic displacement limit of the FST, the PSE stiffness must be increased to obtain a higher stiffness at the critical dynamic displacement. Various stiffness groups for the PSEs and NSEs can provide the same dynamic bearing capacity and yet have a significantly different vibration reduction effect. Excessive stiffness nonlinearity levels cannot effectively improve the vibration reduction effect at the natural frequency. Furthermore, they also significantly amplify the vibrations above the natural frequency. In this paper, the vertical vibration acceleration level (VAL) of the floating slab and the supporting force of the FST can be decreased by 6.9 dB and 55％, respectively, at the resonance frequency.