Abstract: The study aims to investigate the antifungal mechanism of the supernatant fermentation from Z4 strain in vitro. The action mechanism of Z4 strain was surveyed by inhibiting the mycelial growth ratio and spore germination, combining scanning electronic microscope, transmission electronic microscope and potted plant. The results indicated that the fermentation extracts exhibited antifungal activities to mycelium growth of Botrytis cinerea. The fermentation extracts could inhibit the spore germination and the bud growth of Botrytis cinerea. Also the sample had a series of marked morphological and structural alterations of the mycelium under the light microscope and electronic microscope, such as prayer beads swollen of part of mycelium and malforming septa under scan electronic microscope, the cell wall dissolution and the cell liquid seepage under transmission electronic microscope. In addition, disease index of potted cucumber after treated by rootirrigation and spread on leaf were reduced from 82.3 to 28.05 and 83.11 to 30.60, respectively. Control efficiency of the fermentation products of Z4 strain against the Botrytis cinerea were 63.12% and 60.28%, respectively. The cytological changes above were resulted in the necrosis of mycelium cell wall. In addition, the conductivity of the sample displayed in internal of cucumber.