Abstract: In order to explore the inhibiting mechanism of Lactarius vellereus fermenting liquor extraction to the growth of Alternaria alternata(Fr.) Keissler,the influences of the extraction on the activity of protective enzymes in A.alternata were studied,also the correlations among protective enzyme activity,content of MDA,conductivity ratio and variation of respiration intensity were discussed. The results showed that,after being treated with the extraction,the activity of SOD,CAT and POD in mycelium trended increasing in prophase,both SOD and MDA reached the maximum at eight hours,while POD reached at 10 hours.Along with treated time prolonged,the activity of these three enzymes decreased rapidly in anaphase,and dropped to the minimum at 48 hours.However,the activity of POD dropped to zero.These three enzymes were sensitive to the extraction.These results also indicated that the content of MDA reached the maximum at eight hours,which was 9.2 times of the control,and decreased to the minimum at 48 hours,while it was larger than the control all the time.These indicated that the lipid was peroxidized seriously,configuration of membrane system was destroyed,and induced to the leakage of electrolyte,both conductivity ratio and respiration intensity trended decreasing ultimately.These reflected that these three enzymes,the content of MDA,conductivity ratio and respiration intensity varied correlatively after being treated with the extraction.The dynamic balance among SOD,CAT and POD was broken by the extraction,which leading to the free radical clear system undergoing dysfunction,the content of MDA increased and the integrality of membrane system was damaged seriously.So the mycelium was damnified and destroyed.This is probably one of the mechanisms that the extraction inhibites the growth of A.alternata.