In this study,CdS/g-C_3N_4(CSCN) heterojunctions were in situ fabricated with a large amount of Cd S nanoparticles anchored on g-C_3N_4 nanosheets.A wet chemical method was developed for the first time to determine the actual content of Cd S in CSCN composites.X-ray diffraction(XRD),Fourier transform infrared spectra(FTIR),high-resolution transmission electron microscopy(HRTEM) and UV-vis diffuse reflectance spectra(DRS) were employed to characterize the composition,structure and optical property of CSCN composites.Based on the isoelectric point(IEP) analysis of g-C_3N_4,a conclusion was obtained on the combination mechanism between Cd S nanoparticles and g-C_3N_4 nanosheets.The photocatalytic activity of CSCN composites was much better than those of individual Cd S and g-C_3N_4 for the degradation of azo dye Methyl Orange(MO) by 40 min adsorption in the dark followed by 15 min photocatalysis under visible light irradiation.After 5 cycles,CSCN composites still maintained high reactive activity with the MO degradation efficiency of 93.8%,exhibiting good photocatalytic stability.The Cd2+concentration dissolved in the supernatant detected by atomic absorption spectroscopy(AAS) of CSCN composites was lower than that of pure Cd S,implying that the photocorrosion of Cd S could be suppressed via the combination with g-C_3N_4.Photoluminescence emission spectra(PL) results clearly revealed that the recombination of photogenerated electron-hole pairs in CSCN composites was effectively inhibited due to the formation of heterojunctions.Based on the band alignments of g-C_3N_4 and Cd S,the possible photocatalytic mechnism was discussed. In this study, CdS / g-C_3N_4 (CSCN) heterojunctions were in situ fabricated with a large amount of Cd S nanoparticles anchored on g-C_3N_4 nanosheets. A wet chemical method was developed for the first time to determine the actual content of Cd S In high resolution transmission electron microscopy (HRTEM) and UV-vis diffuse reflectance spectra (DRS) were employed to characterize the composition, structure and optical property of CSCN composites. Based on the isoelectric point (IEP) analysis of g-C_3N_4, a conclusion was obtained on the combination mechanism between Cd S nanoparticles and g-C_3N_4 nanosheets.The photocatalytic activity of CSCN composites was much better than those of individual Cd S and g-C_3N_4 for the degradation of azo dye Methyl Orange (MO) by 40 min adsorption in the dark followed by 15 min photocatalysis under visible light irradiation. After 5 cycles, CSCN composites still maintained high reactive activity with the MO degradation efficiency of 93.8%, exhibiting good photocatalytic stability. Cd2 + concentration dissolved in the supernatant detected by atomic absorption spectroscopy (AAS) of CSCN composites was lower than that of pure Cd S, implying that the photocorrosion of Cd S could be suppressed via the combination with g-C_3N_4.Photoluminescence emission spectra (PL) results clearly revealed that the recombination of photogenerated electron-hole pairs in CSCN composites was indeed inhibited due to the formation of heterojunctions. Based on the band alignments of g- C_3N_4 and Cd S, the possible photocatalytic mechnism was discussed.