Poria cocos was cultured on the solid medium using Lentinula edodes stalk as the sole substrate to study whether the mycelia of P. cocos can ferment and utilize L. edodes stalk substrate and improve its nutritional value and biological activity. The activity of key enzymes， the components of main nutrient， and the antioxidant activity of water extracts from the fermented substrate during solid-state fermentation of P. cocos were measured. The results showed that the activity of cellulose exonuclease reached 46.14 U/g at the 6th day of fermentation， then rapidly decreased to 17.15 U/g， and remained basically unchanged at the 18th day of fermentation as the process of fermentation progressed. The activity of carboxymethyl cellulase and xylanase reached 77.70 U/g and 74.23 U/g at the 12th day of fermentation， and then gradually decreased. The activity of β-glucosidase reached 75.06 U/g at the 18th day of the fermentation and then gradually decreased. Compared with the unfermented L. edodes stalk substrate， the content of total sugar， soluble protein， and insoluble dietary fiber in the fermented L. edodes stalk substrate significantly decreased， but the content of reducing sugar， polysaccharides， total phenols， total flavone， amino nitrogen， and soluble dietary fiber significantly increased. The antioxidant activity of water extracts from the fermented substrate was significantly increased. The scavenging rate of DPPH radical and hydroxyl radical increased by 97.75% and 38.15%， compared to the 0 day of fermentation. It is indicated that P. cocos can ferment L. edodes stalk， alter the composition of nutrient in L. edodes stalks， and that the fermented substrate has good ability to scavenge free radicals.