Abstract: Variations on relative activity, sulfhydryl content, fluorescence and UV intensity of mushroom PPO induced by lactic acid were investigated. The results showed that the relative activity of PPO decreased gradually with the increasing of lactic acid concentration, the relative activity of PPO dropped to 1.20% of its original activity at 40mmol/L. The activity of PPO was inhibited significantly by lactic acid at 40mmol/L. By the detection of sulfhydryl content (SH) , the SH content decreased gradually with increasing lactic acid concentration and was only 81.59% at 30mmol/L. The SH content, decreasing from 100% to 80.09%, following residual relative activities, decreasing from 100% to 0.17%, showed a liner relationship, when citric acid increased from 0 to50mmol/L (R2=0.9861) . When PPO was induced by different concentrations of lactic acid, the conformation of PPO was gradually unfolded which induced the decreasing of relative activity.