Novel visible-light photocatalytic nanocomposites composed of cyclized polyacrylonitrile (CPAN) and CdS nanoparticles were prepared using a simple chemisorption and heat treatment approach. The as-prepared nanocomposites were systematically characterized by XRD, SEM, TEM (HRTEM), FTIR, XPS, UV-Vis DRS, PL, EIS, etc., and their visible-light photocatalytic performances were investigated by evaluating the photodegradation of methyl orange solutions under visible-light irradiation. The results show that the as-prepared nanocomposites display sphere-like morphology and their size is about 5 nm. A small amount of CPAN on the CdS surface can significantly enhance the absorption of CdS in the range of 500-800 nm and separation efficiency between the photogenerated electron/hole pairs in the nanocomposites. The investigated nanocomposites exhibit much higher visible-light photocatalytic activity than pure CdS and excellent photocatalytic stability. As the CPAN content increases, the visible-light photocatalytic activity of the nanocomposites increases first and then decreases, and the nanocomposite with the CPAN and CdS mass ratio of 1:50 shows the highest visible-light photocatalytic activity. A possible visible-light photocatalytic mechanism of the CPAN/CdS photocatalysts was proposed.