In this paper, an ultra-thinspiral-structured metamaterial absorber is demonstrated both numerically and experimentally, whose thickness (1.034 mm) is about (1/60th, 1/44th, 1/32th, 1/23th, 1/21th) of the respective working wavelengths (4.81, 6.59, 9.16, 12.69 and 13.71 GHz). Simulation results show that the absorber can achieve absorption up to 94.41%, 99.89%, 99.73%, 99.26% and 99.41% at 4.81, 6.59, 9.16, 12.69 and 13.71 GHz respectively. Thus multi-band absorptions are realized. From the two aspects of surface current and power loss density, the mechanism of strong absorption is analyzed. Theoretical analysis shows that the multi-band absorption arises from multi-order plasmon resonances at five neighboring frequencies. Strong absorptions are induced due to strong electric resonances within/between the spiral structures. Design of the metamaterial absorber is simple and is easy to be implemented, so such absorbers may have application values in designing novel electromagnetic absorbers.