In this study,a susceptible cultivar from the cross of ‘Huacui’ and ‘Golden Delicious’ was used as our material,the interaction process of Alternaria mali and host leaves was examined with scanning electron microscope and transmission electron microscope in order to understand the dynamic changes of A.mali infects host.The results indicated that A.mali usually spread on the surface of host leaves not through stomatal apparatuses or other ways in the infection process.During the late stage,the chloroplast and plasma membrane of host leaves changed clearly,and many other cell organelles disintegrated,indicated that A.mali had excreted AM toxins acted on host cells,and the host cells started to decline.