The invention relates to the preparation domain of plant source pesticide, especially a plant source bactericide and artificial synthesis method thereof, in order to solve that the essential oil in marigold is used as plant source bactericide and applied in plant pathogen. The bactericide is 2,4-bicycloalkene-pentane-cyclopentanone the formula is as figure, firstly the cyclopentanone is heated alkalinity condition to generate 2-bicycloalkene-pentane-cyclopentanone, and then the 2-bicycloalkene-pentane-cyclopentanone reacts with cyclopentanone to generate 2,4-bicycloalkene-pentane-cyclopentanone in the presence of catalyst; the water layer, oil layer are removed by the distillation way to collect volatile component to obtain 2,4-bicycloalkene-pentane-cyclopentanone. The plant source bactericide has features of simple operation method, mild reaction condition, high yield, suitable for large-scale industrialization production, more important, the plant source bactericide is applied for the plant source pesticide, with killing function to the plant pathogen such as watermelon, pepper wilt, Pestalotiopsis sp, Sclerotinia sclerotiorum, with high inhibition rate and provides new path of the development of Chinese plant source pesticide.