This paper presents an novel ultra-wideband thin frequency selective surface (FSS) absorber loaded with lumped resistors. The proposed absorber consists of a single FSS lossy layer with a single resonance structure, and it features thinness, ultra-wide bandwidth and polarization-insensitivity. The absorber is designed with lumped resistors loaded at positions that deviates from the central symmetry axis of the unit cell. It also features the nonuniformly wide metallic strips and the addition of branches with circular tops. All these specific design effectively enhances the bandwidth of the absorber. Both an equivalent circuit model and full wave simulation demonstrate that the proposed absorber achieves over 90% absorption in the frequency range of 6.0-26.77 GHz, with a fractional bandwidth of 126.8%. The thickness of the proposed absorber is 0.086λ_L (λ_L is the wavelength at the lowest frequency), which is only 1.09 times the ultimate thickness based on Rozanov’s theory. A prototype of the proposed absorber is fabricated, and good agreements between experimental and simulated results are observed, validating the effectiveness of the design.