The invention discloses the identification and application of a cotton fibre-specific HB (homeobox) transcription factor gene KNAT7. The cotton fibre-specific KNAT7 belongs to a member of the homeobox (HB) transcription factor gene family, the gene sequence does not have an intron, and the overall length of the cDNA (complementary DNA) of the cotton fibre-specific KNAT7 is 1244bp. A KNAT7 gene-coded protein has 300 amino acids, and the molecular weight of the protein is 33.8kDa. When the KNAT7 is specifically expressed in cotton fibres, a great deal of transcription products of the KNAT7 can be accumulated in fibres at the later stage of cell elongation and the early stage of secondary wall thickening. The expression of the KNAT7 gene in yeast can promote the longitudinal elongation of host cells, and a transgenic arabidopsis analysis shows that the synthesis of secondary wall components is affected by the KNAT7 to a certain degree. The experimental result proves that the KNAT7 plays an important role in the processes of cotton fibre elongation and secondary wall thickening, and the gene has important application value in improving the quality of cotton fibres and increasing the yield of cotton.