Photocatalytic water splitting to generate hydrogen gas is an ideal solution for environmental pollution and unsustainable energy issues. In the past few decades, many efforts have been made to increase the efficiency of hydrogen production. One of the most important ways is to achieve light absorption in the visible range to improve the conversion efficiency of solar energy into chemical energy, but it still presents great challenges. We here predicted a novel organic film, which can be obtained by polymerizing HTAP molecules, as an ideal material for photocatalytic water splitting. Basing on first-principles calculations and Born-Oppenheimer quantum molecular dynamic simulations, the metal-free two-dimensional nanomaterial has proven to be structurally stable and with a direct band gap of 2.12 eV, which satisfies the requirement of light absorption in the visible range. More importantly, the conduction bands and valence bands completely engulf the redox potentials of water, making the such film a promising photocatalyst for water splitting. This construction method through the topological periodicity of organic molecules provides a design scheme for the photocatalyst for water splitting.