Abstract: Yeast mannan was prepared by hot water extraction, and then microwave, ultrasonic wave, mannanase and H₂O₂treatments were used to degrade yeast mannan. The factors that influenced degradation of yeast mannan under the best degradation method were investigated, and the degradation conditions were optimized using orthogonal experiment method. The antioxidant activities of yeast mannan without degradation (YM) and the prepared low-molecular-weight yeast mannan (SYM) were also investigated in vitro. The results showed that H₂O₂ treatment could effectively degrade yeast mannan, and ultrasonic wave treatment had a little degradation of yeast mannan. However, microwave treatment and mannanase treatment hardly degraded yeast mannan. The best H₂O₂-degradation conditions of yeast mannan were the concentration of mannan 30 g/L, H₂O₂ concentration 10%, temperature 95℃ and reaction time 3 h. Under the best H₂O₂-degradation condition, the weight average molecular weights of yeast mannan decreased from 76.52 kDa to 8.07 kDa. The results also showed that SYM had a stronger scavenging ability to hydroxyl radicals and superoxide anions than YM.