Abstract: In this study,the pathogenic bacteria of tomato late pathogen was used as the test bacteria to study the mechanism of action of crude extract and purification of nutgall. Extraction,separation and structure analysis of crude extract of nutgall were carried out,and the results were observed by SEM and TEM,determined the action mechanism by the conductivity measurement. The results showed that:The diameter of bacteriostatic zone of ethyl acetate extraction was(33.9±0.4) mm,which was significantly(P<0.05)higher than other parts. Then a group of monomer flow fraction with diameter of(36±0.2) mm was obtained by column chromatography of ethyl acetate extraction,which was significantly(P<0.05)higher than that of other groups. A monomer with a diameter of(37.3±0.2) mm was isolated by secondary column chromatography,and its structure was identified as tannic acid. The tannic acid against the pathogenic bacteria of tomato late pathogen was observed by SEM and TEM,the results showed that tannic acid from Chinese nutgall had dimpled the spore surface of the pathogenic bacteria of tomato late pathogen,the cell margins became less clear,the cell kernel was significantly shrunken,caused some damage to the cell structure. Tannic acid of Chinese nutgall could change the cell membrane permeability of pathogenic bacteria of tomato late pathogen,its effect was concerned in concentration,the higher of the concentration,the better effect the results would be.