The invention discloses an ultra-wideband acoustic absorber comprising multiple rectangular bars which are identical in cross section and different in height. The multiple rectangular bars are arranged in a comb-shaped structure of which the adjacent spacing distance is identical and height evenly and gradually increases. The bottom parts of the rectangular bars are arranged on a rigid backing. According to the ultra-wideband acoustic absorber, the great absorption effect within the wide band can be achieved without any auxiliary artificial absorbing materials, and the material of the rectangular bars is identified as acoustic rigid material in the air as for the manufacturing material of the structure. Besides, the sound-absorbing structure has no pollution to the environment or any damage to the human body, and sustainable and recycling utilization is facilitated by the rigid structure. The ultra-wideband acoustic absorber is wide in band, environment-friendly, rigid and great in the absorption effect so that the ultra-wideband acoustic absorber has wide application prospect in the fields of sound elimination and noise reduction.