Abstract: In this paper, the suitable submerged fermentation method and antioxidant activity of Dendrobium officinale polysaccharide ( DOP) were studied. The effects of temperature, agitation speed, p H and time on yield of cerevisiae were investigated on the basis of single-factor experiments and orthogonal test for fermentation. The optimum fermentation process parameters were fermentation temperature 28 ℃, p H 4.5, agitation speed 170 r/min, fermentation time 54 h. The total antioxidant capacity, DPPH and ABTS free radical scavenging capacity of Dendrobium officinale polysaccharide has been improved by fermentation, and has very strong dose-response relationship.Average molecular weight of polysaccharide by water extraction and fermentation measured by the GPC-MALLS system showed that it was decreased from 4.234 × 10⁵ to 1.077 × 10⁵ after fermentation.The improvement of antioxidant capacity may be related to the decrease of molecular weight.