Abstract: Chondroitin sulfate oligosaccharides were prepared by chondroitinase degradation. Infrared analysis of chondroitin sulfate before and after degradation and the enzymatic product was analyzed by electrospray ionization mass spectrometry, and the DPPH radicals, hydroxyl radical scavenging activity and reducing power of chondroitin sulfate and the chondroitin sulfate oligosaccharides were evaluated.Infrared spectra showed that enzymatic degradation of chondroitin sulfate had no effect on its basic structure, the results of mass analysis showed that the enzymatic product was chondroitin sulfate disaccharide and tetrasccharide.The antioxidant activities experiments showed that the DPPH radicals, hydroxyl radical scavenging activity and reducing power of chondroitin sulfate oligosaccharide were higher than chondroitin sulfate.It is speculated that the result may be because of the decrease of the molecular weight and the formation of unsaturated double bond in the chondroitin sulfate oligosaccharide, also reducing sugar content increased its activity.