Disclosed is a preparation method of double-layer carbon-paper base paper with high uniformity and gradient pores, including adding carbon fibers to alkali lignin aqueous solution for treatment, controlling the alkali lignin aqueous solution to reach subcritical water conditions, and modifying the carbon fibers by sodium phenol reactive groups under the subcritical water conditions of the alkali lignin, for improving hydrophilic groups on a surface of the carbon fibers and a surface rough structure; further, modifying the carbon fibers to produce carbon nanotube clusters on the surface of the carbon fibers to obtain surface-thermally-modified carbon fibers, and adding the surface-thermally-modified carbon fibers to a dispersant for dispersion, stirring together with auxiliary materials to prepare a mixed slurry, and preparing the mixed slurry into the carbon-paper base paper by wet forming process.