Apple fermentation broth (AFB) is a kind of natural agent which is made from rapid fermentation of physiological dropping fruits, artificial thinning fruits and fruits dropping before harvest. Apple and Chitosan Fermentation Broth (ACFB) is made by apple and chitosan.The objective of the experiment was to explore the mechanism of resistance response to Alternaira alternate f.sp.mali.in apple leaves induced by ACFB. The leaves of 2-year-old apple trees (Malus domestica Borkh.CV. ‘kudowu’)were sprayed with apple fruit broth (AFB), apple and chitosan fermentation broth (ACFB), chitosan(CHN) and water(CK). The disease index,control effect, deposition of reactive oxygen species(ROS), lignin(LIG) and link protein(LIP), contents of the reactive oxygen, and the activities of POD, SOD and CAT were investigated after inoculated by pathogen under the above treatments. The results showed the disease index decreased by 4.3% and 2.4% and control effect increased by 40.7% and 22.2% treated by ACFB and AFB respectively, compared to CK. The frequency of ROS, lignin and cross-linked protein deposition in leaves treated by ACFB increased significantly at 12h after inoculation, and the content of reactive oxygen species of were 30% higher than CK. Two peaks were observed in O₂^-· at 12h and 36h and in H₂O₂ at 3h and 24h after inoculation. Compared to CK, the leaves treated by ACFB showed higher activities of POD and SOD and a lower activity of CAT at 9-72h after inoculation. It suggests that ACFB can effectively induce the resistance to leaf spot in apple leaves, which may relate to oxidative burst and antioxidant metabolism at early stage after inoculation.