Photocatalytic water splitting, using solar energy to obtain hydrogen, is an ideal technology for producing new energy. In the process of photocatalysis, the improvement of the catalytic performance of the catalysts used is a matter of great concern to scientists. So far, there are many problems preventing improvements in photocatalytic performance. In this paper, we propose an atom-doping method, which is an effective method to improve the catalytic performance. We present a type of graphene-like carbon nitride material, whose primitive cell is composed of 12 carbon atoms and 14 nitrogen atoms, so it is denoted as g-C12N14. The energy band, density of states, and optical absorption spectrum of g-C12N14 have been studied using first-principles calculations. From the characteristics of these properties, it is concluded that g-C12N14 can be used as a photocatalyst, but its catalytic performance is low. To improve the catalytic performance, atom doping has been used, which can change the electronic state of the surface to enhance the activity of the photocatalyst. It was calculated that the doped phosphorus and boron system improved the optical absorption range, thus improving the photocatalytic efficiency. To make the results more accurate, all the calculations used the computationally-large HSE06 hybrid functional method.