Abstract: OBJECTIVE To investigate the structual composition and in vitro antioxidant activity of crude polysaccharide from different parts of flower of Hylocereus undatus. METHODS The crude polysaccharide from petals, calyx, pistil and stamens in 4 different parts of flower of Hylocereus undatus were extracted by method of water extraction and alcohol precipitation. Structural characteristics of crude polysaccharide from different parts of flower of Hylocereus undatus were investigated and compared using phenol-sulfuric acid method, coomassie brilliant blue method, size exclusion chromatography(SEC) coupled with multiangle laser light scattering and (MALLS) refractive index detector(RID) (SEC-MALLS-RID), HPLC and Fourier transform infrared spectrometer(FTIR), respectively. The 2,2'-azinobis(3-ethylbenzthiazoline-6-sulphonate acid) radical, hydroxyl radical and superoxide anion radical scavenging activities was assayed to evaluate the antioxidant activity in vitro of crude polysaccharide from different parts of flower of Hylocereus undatus. RESULTS The results demonstrated that the contents of polysaccharides were 52.20%-61.52%, the protein contents were 1.17%-3.21% in crude polysaccharide of different parts from Hylocereus undatus. All polysaccharides contained D-mannose, L-rhamnose, D-glucuronic acid, D-galacononic acid, glucose, D-galactose, D-xylose and D-arabinose, but the proportion of monosaccharides varied remarkably in the crude polysaccharides from different parts of flower of Hylocereus undatus. The SEC chromatograms, the molecular weights and their distribution, and the FTIR chromatograms of crude polysaccharides from different parts of flower of Hylocereus undatus were similar. The crude polysaccharide from different parts of flower of Hylocereus undatus had significant anti-oxidative activities in vitro. CONCLUSION The structual composition of crude polysaccharide from different parts of flower of Hylocereus undatus have high similarity and significant anti-oxidative activities in vitro, and can be a resource of health foods and medicine for further development.