The invention discloses a terahertz wave absorber, a preparation method, and an application. The absorber comprises an antenna array, a dielectric layer and a metal layer, wherein the antenna array, the dielectric layer and the metal layer are sequentially arranged from the top to the bottom. The antenna array comprises a plurality of antenna units which are arranged in an array. Each antenna unit comprises four antenna structures which are in rotational symmetry, and the antenna structures are concave structures which are not closed, wherein the openings of the antenna structures are located at the left sides of the concave shape. The thickness of the dielectric layer is the phase difference pi between the reflection light reflected by the antenna array and the reflection light reflected by the metal layer. Because the antenna structures are concave structures which are not closed, the effective lengths of the antenna structures can be increased under the condition that the antenna structures occupy a small area, thereby achieving the absorption of the light at the terahertz band. The thickness of the dielectric layer enables the reflection light reflected by the antenna array to be counteracted by the reflection light reflected by the metal layer, thereby improving the absorptivity of the absorber.