Pumpkin polysaccharides were extracted with hot water and deproteinized by Saveg method. After rehydration with distilled water, soluble polysaccharides were successively fractionated by different gradient concentrations of ethanol（0～30%，30%～60%，60%～80%）into three precipitation fractions, named PSP-30, PSP-60 and PSP-80. Contents of sugar, protein and monosaccharide composition were determined, UV and IR spectroscopy analysis were run simultaneously to determine their spectroscopy properties. The results indicated that all three fractions were combined with a certain amount of protein, and uronic acid was not found. PSP-30 and PSP-60 were composed of mannose, arabinose, xylose, mannose, glucose and galactose, but PSP-80 was composed of mannose, arabinose, xylose, glucose and galactose. Each of these fractions all had good scavenging effect on DPPH free radical, and PSP-60 removal rate was the highest, its removal rate was 61.90% at concentration of 2.5 mg/mL, PSP-60 was treated with different frequencies ultrasonic, the mol ratios of these monosaccharides and antioxidant activity were changed. The results confirm that ultrasonic can cause monosaccharide transformation and biological activity change of polysaccharides. However, there is no regularity between the increase of ultrasound frequency and the monosaccharide composition after transformation. So the effect of ultrasound on polysaccharide structure is still a task to be studied.