In light of the escalating severity of water pollution, the development of innovative and efficient photocatalytic materials has emerged as an urgent requirement. In this study, P-doped ultrathin g-CN was successfully synthesized using the chemical vapor deposition technique, specifically to enhance the photocatalytic degradation of the antibiotic tetracycline hydrochloride (TC) and the dye rhodamine B (RhB). The purpose of P doping was to modify the electronic structure of g-CN and increase its photocatalytic efficiency. The microstructure, chemical state, and light absorption properties of the material were extensively analyzed using advanced characterization techniques such as high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS). The study revealed that P doping not only enhanced the structural stability of the nanosheets but also effectively broadened the material's light-responsive range by creating new band structures. Photocatalytic tests showed that under visible light irradiation, P-doped ultrathin g-CN demonstrated outstanding degradation efficiency against TC and RhB. Specifically, the doped catalyst reduced the concentrations of TC and RhB by more than 90% within 60 minutes, markedly surpassing the performance of undoped graphitic carbon nitride nanosheets. Moreover, the phosphorus-doped materials retained high catalytic activity and stability after repeated usage cycles. From a theoretical perspective, this research applied density functional theory (DFT) based on first principles to deeply investigate the electronic properties of P-doped ultrathin g-CN. The calculations disclosed that phosphorus doping led to modifications in the band structure, particularly an increase in localized states at the valence band top, which is vital for facilitating the effective separation of photogenerated electron-hole pairs. Additionally, the doping significantly altered the charge distribution on the material's surface, enhancing the catalyst's capacity to adsorb and activate molecules of TC and RhB. Overall, this research not only provided an efficient photocatalytic material for addressing persistent pollutants in aquatic environments but also explored the fundamental mechanisms by which P doping improves the photocatalytic performance of g-CN, thereby offering theoretical and experimental bases for the design of novel photocatalysts.