The work aims to design and prepare a flexible Frequency Selective Surface absorber with matching unit conductivity and excellent electromagnetic properties. Firstly, the functional conductive sheet with good conductivity was prepared by Spatial Confining Forced Network Assembly (SCFNA) and then cut into array according to the round resonant unit and encapsulated in the organic dielectric layer. The electromagnetic wave performance of the Frequency Selective Surface absorber was tested and compared with the optimization results of simulation model. The absorptivity of Frequency Selective Surface samples was over 96.22% in 18~40 GHz. Spatial Confining Forced Network Assembly is used to prepare the conductive unit, and the precise regulation of the unit conductivity is achieved by adjusting the process parameters. Thus, a highly flexible absorber with excellent electromagnetic properties in the 18~40 GHz band is successfully prepared.