With the increasing operational speeds of high-speed trains, suppressing low-frequency floor vibration to enhance passenger comfort has become a critical challenge. Conventional passive isolators are often ineffective in the human-sensitive low-frequency range (20–50 Hz) and struggle to balance static load-bearing with dynamic isolation. This paper proposes a novel Inclined-Beam Quasi-Zero-Stiffness (IB-QZS) isolator to address this issue. The isolator features an innovative parallel configuration of a positive-stiffness fish-belly-shaped outer frame and a negative-stiffness mechanism composed of double-layer inclined buckled Euler beams. An analytical model based on the elliptic integral method was developed to describe its nonlinear force-displacement characteristics, and its high-static-low-dynamic-stiffness (HSLDS) behavior was verified through Finite Element Method (FEM). A prototype was then fabricated and tested. Experimental results demonstrate that the isolator supports loads from 5.1 kg to 35.8 kg with a static displacement under 1.2 mm. The measured force-displacement curve confirms the QZS characteristic within the operational displacement range of 0.3–1.2 mm, showing excellent agreement with theoretical and simulation predictions. Furthermore, comparative dynamic tests demonstrated that the IB-QZSI offers significantly superior vibration isolation performance compared to conventional rubber isolators. The findings validate that the proposed IB-QZS isolator is a promising solution for achieving superior low-frequency vibration attenuation in high-speed train floor structures, offering significant potential for improving ride comfort.