A three-layer solar ultra-wideband absorber is designed based on a t hree-gratings cascade top-layer structure with different materials and sizes.The effects of the thickness of chromium film,Th e refractive index and thickness of buffer layer,the period of absorber cell and the ratio of width to height of tri-grating on the absorption spectrum bandwidth and absorption rate were numerically simulated by using the finite-difference time-domain method ( FDTD).The physical mechanism of wide spectrum and high absorptivity is investigated by selecting the electromagnetic field distribution at any wavelength and combining the local Surface Plasmon resonance.The simulation results show that three sing le-grating cascades with different material and structure parameters can obviously broaden the absorption spectrum bandwidth of the incident light,A ultra-wideband was obtained including of ultraviolet (UV),visible light (VIS) and infrared (IR). The absorption spectrum was as wide as 2.2μm,and the absorption of near 1μm was perfect.The absorber can still maintain good ab sorption performance and polarization sensitivity in a wide range of incident angles.The absorber designed in this paper is simpl e in structure,small in size and easy to be integrated with chip.It can be used in photovoltaic generation,solar heat trea tment and photo detection.