A hybrid metasurface combining spoof surface plasmon polariton (SSPP) absorber based on a slow-wave mechanism and a polarization conversion metasurface (PCM) based on a fast-wave mechanism is proposed for ultra-wideband radar cross-section (RCS) reduction. The design aims to overcome the bandwidth limitation of a single RCS-reduction mechanism and to mitigate the mutual interference between different functional structures in conventional hybrid metasurfaces. First, a broadband SSPP absorber based on a slow-wave dissipation mechanism and a broadband PCM based on a fast-wave phase-cancellation mechanism are designed. The metasurface aperture is then divided into slow-wave and fast-wave regions, where the SSPP absorbers and chessboard-arranged PCM subarrays are respectively integrated. In this manner, the two mechanisms are spatially integrated and their operating bands are complementarily combined in the frequency domain. For the SSPP absorber, incident electromagnetic waves are strongly confined as slow waves and dissipated through dielectric and resistive losses. For the PCM, the incident waves are converted into orthogonal-polarized reflected waves, whose fast-wave scattering is suppressed by phase cancellation. More importantly, the distinct propagation characteristics of the slow and fast waves suppress energy exchange between the two functional regions, thereby reducing mutual interference, shadowing effects, and near-field coupling while preserving their respective RCS-reduction mechanisms. Numerical results demonstrate that the proposed hybrid metasurface achieves more than 10 dB RCS reduction over an ultra-wide frequency range of 3–25 GHz, with a maximum reduction of 35 dB. Experimental results further confirm the effectiveness of the proposed design. By combining slow-wave absorption and fast-wave phase cancellation in both the spatial and frequency domains with weak mutual interference, this work provides an effective approach to the design of ultra-wideband and high-performance low-RCS metasurfaces for electromagnetic stealth and electromagnetic compatibility applications.