A novel conidium development mutant was obtained by screening the transformants of Botrytis cinerea produced by Agrobacterium tumefaciens mediated method, which lost the ability of producing conidia. The flanking sequence of T-DNA insertion site was acquired by TAIL-PCR technology, and then, the T-DNA insertion in the second exon of BC1G_02800.1 confirmed by BLAST between the flanking sequence and the known sequence in the B. cinerea gene database. The mutant gene was identified as BC1G_02799.1 located in the upstream of BC1G_02800.1 gene by RT-PCR. The DNA full-length sequence of BC1G_02799.1 was 1951 bp and contained 1848 bp coding region, which encoded a 615 amino acids putative protein similar to ABC-transporter, and the function of BC1G_02799.1 gene was unknown to date. Phenotype analysis of the mutant found that the mutant strain colony was white, grew slowly, and did not produce conidium and sclerotia on PDA medium but showed a stronger pathogenicity to tomato leaves and successfully increased the enzyme activity related to pathogenicity compared to the wild type strain. The results suggested that the BC1G_02799.1 gene was involved in the conidium development, the sclerotia formation, and pathogenicity in B. cinerea. Our research will facilitate in understanding the molecular mechanism of conidium development, sclerotia formation, and pathogenic in B. cinerea.