The influences of extract from Lactarius vellereus fermentation liquid on antioxidant enzymes activity of Alternaria alternata K. were evaluated. Results showed that, in treatment group, the activities of SOD, CAT and POD in mycelia of A. alternata increased at prophase, and SOD activity increased to maximum (31.562 U·g-1) at 5h, while both CAT and POD activity increased to maximum (0.041U·g-1·s-1 and 5.531U·g-1·min-1, respectively) after being administered with 10h. The activities of these three enzymes treated with extract decreased rapidly at anaphase and down to the minimum (0.828 U·g-1, 0.012 U·g-1·s-1, and 0 U·g-1·min-1, respectively) at 40h. While, in control group, SOD, CAT and POD activity always increased with treatment time prolonged. The activity of SOD, CAT and POD was affected by Lactarius vellereus fermentation liquid extract so that it led to dysfunction of the free radical scavenging system. This might be one of the mechanisms that the extract inhibits the growth of A. alternata.