A shared aperture metamaterial (SA-MTM) with partially reflection and absorber characteristics is presented. The SA-MTM is composed of two metallic layers separated by a dielectric spacer; the top absorbing surface (AS) consists of oblique cross metallic pattern loaded with lumped resistances, and the bottom partially reflecting surface (PRS) consists of etched parallel slots in a metallic layer. An SA-MTM with partial reflection and absorption characteristics is fulfilled by making the absorbing surface and partially reflecting surface shared the same aperture in the vertical direction. The SA-MTM is applied to the waveguide slot antenna; the Fabry-Perot resonance cavity constructed by the PRS and the metallic ground layer of the waveguide slot antenna can achieve high gain, while the AS can obtain the low radar cross section (RCS) characteristic antenna by absorbing the incident wave. Simulation and experimental results demonstrate that the antenna with SA-MTM gain is enhanced above 3 dB in the operation frequency range, the backscattering RCS is obviously reduced in a frequency range of 2–9 GHz. This idea can help us design a high gain and low RCS antenna, which overcomes the conflict between scattering and radiation performance of antenna.