We propose a method of designing ultrathin broadband perfect metamaterial absorber (PMA) which is based on the parameters of the cell. The bandwidth is enhanced via the method which combines the multilayer and multi-resonance in a layer. And it is not complex due to having no lumped elements in it, so it is easy to fabricate and apply. In order to illustrate the method, a double-layer perfect metamaterial absorber with three resonance peaks is designed using the above method. The equivalent circuit of the proposed absorber is analyzed so as to better understand the mechanism of the high absorption. By adjusting geometric parameters of the structure, we can obtain a polarization-insensitive and wide-incident-angle ultra-thin absorber. Simulated and experimental results show that the full-width at half-maximum is 14.1% when the thickness of the filer is only 0.01λ, and the bandwidth of-3 dBsm radar cross section reduction is 18.9%. At resonance, the reduction value may exceed 23 dBsm while the absorber has a good characteristic of RCS reduction at the boresight direction from-40° to +40°.