Insect cytochrome P450 monooxygenases play an important role in plant allelochemical detoxification. In this study, a full-length gene CYP305A1 of the P450 Clan 2 family was cloned from Aphis gossypii Glover, and its promoter was identified and characterized. The transcript level of CYP305A1 and its promoter activity were significantly induced by two plant allelochemicals, gossypol and 2-tridecanone. Furthermore, the 5′-end promoter region from -810 to +62 bp was demonstrated to be essential for basal transcriptional activity of CYP305A1, and the promoter region from -810 to -581 bp was shown as an essential plant allelochemical responsive element and had a cis-element 5′-CACACTA-3′ as the binding site of aryl hydrocarbon receptor. Interestingly, there was an identical overlapping region of 1,094 bp between CYP305A1 promoter and the venom protease gene. When the expression of CYP305A1 gene was knocked down by RNA interference with CYP305A1 dsRNA, the expression of the venom protease gene was decreased. However, the knockdown of the expression of the venom protease gene did not affect the CYP305A1 expression. These results provide important insights for understanding the functions of P450 genes and the regulatory mechanism of P450 gene expressions in the resistance of Aphis gossypii Glover to plant allelochemicals.