A novel broadband absorber in microwave frequency band based on metamaterials is designed, which is a typical three-layer structure with top and bottom layers of metal and the middle layer of dielectric. The top layer is a metal structure which is designed on a single cell by using the principle of multiple-absorption-peak superposition and composed of a circular open ring and a square open ring. The simulation results show that the absorptivity of the proposed absorber is over 90% in the frequency range of 10.65-22.39 GHz, the full width at half maximum (FWHM) is 13.07 GHz, and the relative FWHM is 83.7%. The overall thickness of this proposed absorber is only about 1/10 of the center wavelength.