This article proposes a phase compensation method for oblique incident electromagnetic wave to improve the angular stability of frequency selective surface (FSS) absorber. By using this method, an ultra-wideband, incident angle-stable FSS absorber-based electromagnetic shield structure is designed to reduce the electromagnetic environment level in enclosed cabins. The proposed single-layer FSS absorber achieves excellent angular stability within an ultrawide band by intentionally using different dielectric layers to compensate for the electromagnetic wave phase at high and low frequency bands respectively, and by designing a novel FSS cross unit cell featuring with gradually width-varying, slotted and top-loaded metallic strips. Simulation results reveal that the proposed absorber achieve over 90% absorption in the frequency range of 3.9-25.8 GHz, with a fractional bandwidth of 147.5%. In the frequency range of 4.7-22.1 GHz (129.9%), the angular stability of two polarizations reaches 30° with 90% absorptivity, and the absorptivity retains over 80% even when the incident angle increases up to 50°. The good agreement between the measurement and simulation results has verified the effectiveness of the design.