Abstract: Due to some deficiencies of the polysaccharides degradation by the methods of chemical，thermal or enzymatic treatment，sonication was introduced for the degradation of Porphyra yezoensis polysaccharide（PYPS）.The Mark-Houwink equation，showing the relationship between molecular weight and intrinsic viscosity，was set up.It was found from the results of the degradation dynamics that there was an exponential relation between the rate constant and treatment time of sonication，different from the traditional chemical，thermal or enzymatic treatment.The activation energy of the degradation was 52.13 kJ·mol-1，and it could be preliminarily concluded that ultrasonic degradation was better than the enzymatic ones.