The invention discloses a simple chemical vapor deposition (CVD) method for synthesizing a g-C3N4 ultra small quantum-dot modified titanium oxide nanotube catalyst. According to the invention, the previously synthesized titanium oxide nanotube array is used as the base, a precursor is firstly placed in a crucible, through adopting the high temperature treatment method, the precursor is enabled to become steam, and deposited in the titanium oxide nanotube, then the precursor is condensed into nucleuses in the titanium oxide nanotube to form g-C3N4 of the quantum dot. Due to the obtained catalyst in the visible light, the material is enabled to have the hydrogen producing performance in the visible light by utilizing the photoelectrocatalysis device. According to the invention, the CVD method is adopted to calcine materials in a muffle furnace to obtain the photocatalyst with visible-light response, and the catalyst can show the excellent visible light photocatalytic activity under the effect of photoelectrocatalysis. The catalyst is simple in preparation method, can be produced in large batch, causes no environment pollution during the preparation process, and can be widely applied to the fields of visible regions of the sunlight, solar cells, antibiosis, photocatalysis treatment of pollutants and the like.