<p>Cultural relics are indispensable carriers of human civilization, yet their seismic protection remains a major challenge during storage and exhibition. This study proposes a new type of guide-rail isolation device for seismic protection of cultural relics. The device consists of two orthogonally stacked unidirectional horizontal isolation systems, adopts laterally arranged tension springs to achieve variable stiffness, and uses a lead-screw–rotary-damper mechanism as an independent energy-dissipation component. Prototype tests and numerical simulations were conducted to identify the basic mechanical properties of the device, evaluate its operational reliability and repeatability, and assess its isolation performance under selected test conditions. The results show that the prototype has a low motion-initiation threshold, weak coupling between the two orthogonal isolation layers under the adopted test configurations, and stable and repeatable motion. Under three selected ground motions scaled to PGA = 0.4 g, the isolation efficiency reached 72%–84%, and the residual displacement was less than 2.4 mm, indicating good recentering performance. The established numerical model showed reasonable agreement with the experimental results and can support subsequent parametric analyses. The variable stiffness design increases the device stiffness with displacement; when combined with appropriate damping parameters, it helps coordinate acceleration suppression and displacement limitation and shows potential adaptability to different seismic intensity levels within the scope of the prototype tests and numerical analyses.</p>