As an excellent semiconductor material, GaN has a wide band gap, which causes it to only absorb violet light in visible light. Therefore, how to increase the utilization rate of visible light of GaN material is a problem worthy of study. Doping is commonly used to solve this problem. Therefore, this paper uses the first-principles method to calculate the electronic structure and optical properties of the intrinsic GaN, C single-doped, Ti single-doped, and C-Ti co-doped GaN. The results show that the stability of the system after doping is all good; the volume of each system increases after doping, indicating that the introduction of impurities causes distortion of the system lattice, which helps to promote the separation of photogenerated hole-electron pairs, and further improves the photocatalytic performance of the material; the energy level of the system is split and the electronic transition is easier after the introduction of impurity elements; after doping, the main peak of the imaginary part of the dielectric function of each system moves to the low-energy region, and the absorption spectrum is red-shifted to the visible light region, and the co-doped system is in the blue-green light region, and the absorption coefficient is the largest, so it can be inferred that C-Ti co-doping will help improve the photocatalytic performance of GaN.