The invention discloses a fusarium oxysporum f.sp.niveum RNAi component FonDCL1 gene deletion mutant and a construction method thereof. According to the fusarium oxysporum f.sp.niveum RNAi component FonDCL1 gene deletion mutant and the construction method thereof, according to a reported Dicer like 1 protein sequence, homologous alignment is conducted on a Fon genomic sequence to obtain a fusariumoxysporum f.sp.niveum FonDCL1 gene, by adopting a homologous gene substitution principle, by means of a Split-marker strategy, the target gene FonDCL1 is substituted for a hygromycin B (HPH) gene DNAfragment, and by constructing a homologous gene recombinant fragment, through genetic transformation of a wild strain Fon1, the FonDCL1 gene deletion mutant is obtained. The FonDCL1 mutant has the advantages that the virulence to watermelon seedlings is enhanced, and the FonDCL1 mutant is more sensitive to abiotic stress factors of monensin sodium salt, hydroxycarbamide, a fluorescent whitening agent CFW, Congo red (CR), hydrogen peroxide and the like, so that a research foundation is provided for functional assignment of small RNA generated in processes of self growth and development and watermelon infection of the Fon1.