Objective： The phenolic substances in the extract of Agrocybe chaxingu Huang and its effect on antioxidant activity in vivo and in vitro were studied. Method： Based on the analysis of total phenols， total sugars and antioxidant activities in vitro of water/ acetone/ ethanol extract of A. chaxingu， the antioxidant activity of A. chaxingu aqueous extract [total phenols， 12 mg/（kg bw·d）] on normal mice （gavaged for 10 days） was detected， which made ferulic acid and vitamin E as positive control group， and physiological saline as blank control group. Then， the antioxidant activity of A.chaxingu aqueous extract [based on total phenols， 10， 20， 40 mg/（kg bw·d）] on the D-galactose induced aging mice was evaluated. Result： The content of total phenols of A. chaxingu aqueous extract was 1.25‰， which between acetone extract and ethanol extract. When the concentration of A. chaxingu aqueous extract was 60.00 mg/mL， the scavenging rate of hydroxyl radicals and the inhibition rate of polyunsaturated fatty acid （PUFA） were 87.00% and 65.31%， respectively， which were more higher than acetone extract and ethanol extract. The results of antioxidant activity on normal mice showed that the A. chaxingu aqueous extract did not affect the weight growth， but increased the liver index and spleen index. The activity of SOD in serum of A. chaxingu aqueous extract increased and reached 130.43 U/mL， while the content of MDA decreased obviously compared with the physiological saline group （P<0.05）. Antioxidant capacity of ， aqueous extract was equivalent with vitamin E and ferulic acid. The results of antioxidant activity on D-galactose induced aging mice showed that 10， 20， 40 mg/（kg bw·d） of A. chaxingu aqueous extract could significantly decreased the content of MDA （P<0.01）， and the decrease rate of MDA of 20 mg/（kg bw·d） group was the highest and reached 94.53%. The GSH-Px activity of 10 mg/（kg bw·d） group increased extremely significantly 22.98% （P<0.01）. Conclusion： The A.chaxingu aqueous extract had antioxidant activity in vivo and in vitro， and phenolic substances were the main functional components. A. chaxingu aqueous extract could maintain the body's antioxidant homeostasis and significantly amelioration the D-galactose induced aging damage by increasing the activity of antioxidant enzymes （SOD， GSH-Px） and reducing the content of MDA.