In order to reduce the radar cross section (RCS) of antenna, a wideband-enhanced ultra-thin metamaterial absorber is designed by reducing the distance between the two absorption peaks due to the double resonances. The absorber is composed of two metallic layers separated by a lossy dielectric spacer. The top layer consists of a single-square loop with four splits on the four sides and a square metal patch in the center and the bottom one is of a solid metal. A dipole resonance and an LC resonance are caused by the structure of the metamaterial absorber. By fine adjusting geometry parameters of the structure, we can obtain a polarization-insensitive and wide-incident-angle ultra-thin absorber whose absorption values are 91.6% and 96.5%. On condition that thickness is less than 0.01λ the absorber has a full-width at half-maximum of 8.2%. The absorber is applied to the circularly polarized tilted beam antenna for reducing RCS. Simulated and experimented results show that the RCS reduction of antenna is above 3 dB within the operation band from 5.5 GHz to 6.5 GHz, the gain is not changed and the bandwidth is increased due to the improvement of axial ratio. At the resonance, the most reduction values exceed 8 dBsm and 11 dBsm while the absorber has a good characteristic of RCS reduction at the boresight direction from -36° to +36°.