In recent years, the polymers represented by macromolecular materials have attracted widespread attention due to their higher flexibility and viscoelastic, compared with other materials used for light absorption (such as semiconductor materials, carbon-based materials and noble metal nanomaterials). Although the polymers have shown potential applications in the photothermal field, compared with other light-absorbing materials, the polymer substrates have a low light absorption rate and a narrow absorption bandwidth concentrated in the visible light band. Therefore, it is necessary to prepare a structure on the polymer material layer for absorbing light, thereby improving the ability of the polymer to absorb light. In addition, since the existing preparation processes of polymer absorption structures require the use of templates and the processes are relatively complicated, there is an urgent need for a simple and easy process to prepare the absorption structures on the polymer material layer. In this article, composite nanoforests are prepared on polymer substrates based on a plasma repolymerization technology and magnetron sputtering process; due to the metallic nanoparticles existing, multi-hybrid plasmonic effect is achieved, thus the average light absorption rate of the polymer in a wavelength range of 380–2500 nm is increased from 23.34% to 74.56%. Such polymer composite nanoforests have high absorption characteristics in a wide spectral range. The method of preparing the structure is quite simple, and can be applied to preparing different polymer materials. Besides, by changing the plasma bombardment time, the morphology of the nanoforests can be adjusted; by increasing the size of the metallic nanoparticles, the absorption of the composite nanoforests can be increased. It is foreseeable that the polymer composite nanoforests will have applications in various optical devices.