The invention discloses a dicoumarin derivative and a preparation method and application thereof. The molecular formula of the dicoumarin derivative is C22H18O7, wherein the structural formula is as shown in a formula I. The preparation method of the dicoumarin derivative comprises the following steps: (1) carrying out bacterial culture on aspergillus clavatus (Aspergillus clavatus) R7 in a strain medium; and (2) carrying out fermentation cultivation on the strain obtained in the step (1) in a fermentation medium and then extracting by using an ethanol water solution or a methanol water solution, carrying out vacuum concentration, extracting by using an organic solvent and finally further purifying through silica-gel column chromatography, Sephdex LH-20 gel column chromatography and recrystallization to obtain the dicoumarin derivative. The prepared dicoumarin derivative has very strong effects of inhibiting tomato blight, penicillium italicum and gloeosporium musarum and can be used as a microbial bactericide, the raw materials can be produced on a large scale and the application prospect is wide. The formula is as shown in the specification.