Abstract: In order to study the difference of antioxidant activities among the extracellular polysaccharide (POFP), mycelium polysaccharide (POMP) and fruiting body polysaccharide (POP) of Pleurotus ostreatus by liquid fermentation, this study prepared POFP and POMP through liquid-state fermentation of edible fungi, polysaccharide isolation and purification, as well as other techniques, followed by a comparative evaluation of their antioxidant activities with POP. The study assessed the in vitro antioxidant activity of the three polysaccharides using DPPH radical, ABTS⁺ radical, and ·OH radical scavenging activities as well as total reducing power. Additionally, yeast cell models subjected to ultraviolet (UV)-induced oxidative damage and H₂O₂-induced oxidative damage were employed to evaluate their protective effects against oxidative injury. The results showed that the three kinds of polysaccharides were composed of two kinds of components with different molecular weights, which ranged from 375.31~9788.14 Da, and the order of molecular weight was POP>POMP>POFP. All three polysaccharides demonstrated significant scavenging activities against DPPH, ABTS⁺, and ·OH radicals, along with notable reducing power, all of which exhibited a dose-dependent manner. Among them, POFP displayed the strongest total reducing power, achieving an OD₇₀₀ value of 0.51±0.04 at a concentration of 5 mg/mL, which was higher than that of POMP and POP. Furthermore, all three polysaccharides significantly increased the survival rates of yeast cells subjected to UV- and H₂O₂-induced oxidative damage. Among them, POP demonstrated stronger protective effects, restoring the viability of UV-damaged yeast cells to 96.05%±4.02% at a concentration of 25 mg/mL, and that of H₂O₂-damaged yeast cells to 91.6%±3.26% at 15 mg/mL, which was comparable to the effects observed in the vitamin C (V_C) treatment group. In summary, Pleurotus ostreatus polysaccharides from different sources exhibited significant antioxidant activities, and liquid fermentation technology provided a new way for the preparation of new antioxidant natural products.