Chitosan-CdS composite nanoparticles were prepared by reverse microemulsion reactions. The alizarin red was degraded rapidly by chitosan- CdS composite nanoparticles in a short time. 98.1% of the alizarin red disappeared at 30min when the concentration is 20mg/L. The absorption peak became weak rapidly and disappeared finally at maximum absorption wavelength 261nm during the photocatalytical elimination process of alizarin red, and the new absorption peaks appeared at 223nm and 228nm respectively, which showed new products formed. The elimination of alizarin red was pH-dependent, more efficient at acidic condition than at neutral and alkaline. The photocatalytical elimination efficiency of chitosan-CdS composite nanoparticles was increased by 24.2% and 28.4% respectively at 2min and 30min compared with common CdS. The hypothetical mechanism of photocatalysis was put forward preliminarily: The primary step was the sorption of chitosan-CdS composite nanoparticles, which stimulated the photocatalytical elimination of alizarin red.