Abstract: In this study, the polysaccharides from Dendrobium densiflorum were used to investigate the changes in molecular distribution, intrinsic viscosity, solution conformation and immunoregulatory activity after usltrasonic treatment.High performance gel permeation chromatography indicated that the molecular weight of the polysaccharide was significantly decreased by usltrasonic treatment, which led to a drop in intrinsic viscosity.Atomic force microscope showed that with ultrasonic treatment, the conformation of D.densiflorum polysaccharides was changed into the dispersed state from the large spindle- shaped state.Using in vitro splenic cell proliferation experiment, the immunoregulatory activity of the polysaccharides was enhanced by ultrasonic treatment.These results suggested that the physicochemical properties and biological activities of D.densiflorum polysaccharides could be changed by the ultrasonic treatment, which should be applied to the extraction of natural polysaccharides with caution.