Abstract: Objective: Pitaya stem polysaccharide (PSP) was applied as raw materials to study the physicochemical properties as well as antioxidant activity and its stability. Methods: The active polysaccharide from pitaya stem was extracted using hot-water extraction combining with alcohol-precipitation method. Initially, total sugar, total flavonoid and total phenolic contents in PSP were determined using phenol-sulfuric acid method, NaNO₂-Al(NO₃)₃-NaOH method and Folin-Ciocalteu assay, respectively. Monosaccharide composition of PSP was analyzed using ion chromatography, and surface morphology in PSP was characterized using scanning electron microscope (SEM). Then, antioxidant stability of PSP was systematically determined by simulating various food storage and processing conditions, and hydroxyl radical scavenging activity was used as an evaluation index for PSP. Results: The extraction yield of PSP was around 7.22%, total sugar, total flavonoid and total phenolic contents in PSP were 334.63mg/g, 5.91 mg/g, and 5.05 mg/g, respectively. The PSP mainly consisted of glucose, galactose, rhamnose, galacturonic acid, arabinose and mannose in the following molar ratios: 5.27:2.77:0.77:0.71:0.32:0.17. PSP presented a compact and porous surface with irregular shape as recorded by SEM images. Additionally, PSP possessed good hydroxyl radical scavenging activity, which was increased with the increase of polysaccharide concentration (P<0.05). Temperature, pH value, some metal ions (i.e. Ca²⁺, Na⁺ and K⁺ metal ions), some food additives (i.e. citric acid, sodium benzoate and glucose) and sterilization methods had little impact on the antioxidant activity of PSP. However, light had a great effect on the antioxidant activity of PSP. With the extension of light time, the inhibitory ability of PSP to hydroxyl radical gradually decreased, indicating that PSP had the poor light resistance. Meanwhile, the antioxidant activity of PSP was also significantly decreased with the increasing concentration of Fe³⁺ and Cu²⁺ metal ion, as well as sucrose solution (P<0.05). Conclusion: PSP has good antioxidant stability, but continuous light, Fe³⁺ and Cu²⁺ metal ions, as well as sucrose might have the greatly adverse effect on the antioxidant activity of PSP. Therefore, the direct contact with these four environmental factors should be avoided during the processing and storage of pitaya stem.