The P-doped g-C3N4 (P-CN) has been synthesized by one-step method using diammonium hydrogen phosphate and urea. Then, the CdS/P-CN has been successfully prepared by using P-CN, thioacetamide and cadmium acetate with a hydrothermal method. The crystal structure and surface morphology of CdS/P-CN are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). X-ray photoelectron spectroscopy (XPS) indicates the presence of P-N bonds, Cd2+ and S2- in the nanocomposites. UV-visible spectra demonstrate the absorption range of visible light of the materials increased. The photocatalytic properties of CdS/P-CN are characterized by degradation of Rhodamine B under visible light which exhibits high photocatalytic degradation ability. At the end of the paper, a photocatalytic mechanism has been obtained based on the experimental results. The enhancement of photocatalytic properties was attribute to the doping of P and forming heterojunction, which changes the electronic structure of g-C3N4 and restrains the recombination rate of photogenerated electron-hole pairs.