Cotton is a vital economic crop, and cotton fiber serves as the primary raw material for the textile industry. Lignin in cotton fiber is closely associated with fiber quality. Lignin is synthesized through the phenylpropanoid metabolic pathway, where the cinnamyl alcohol dehydrogenase gene CAD6 plays a significant role. In this study, we obtained successfully transformed overexpression plants by constructing an overexpression vector and performing genetic transformation and tissue culture. To verify the function of the GhCAD6 gene in upland cotton, we analyzed the agronomic traits, fiber quality, cell wall structure, and lignin content of GhCAD6-overexpressing plants. Our results indicate that the GhCAD6 gene is predominantly expressed during the stages of fiber elongation and secondary wall synthesis. Overexpression of the GhCAD6 gene resulted in increased plant lignin content and fiber upper half mean length, boll number per plant, fiber uniformity index, strength, and lint were improved. The fiber surface was smoother, and the fiber cell wall was more compact. These findings demonstrate that the GhCAD6 gene positively regulates lignin synthesis and fiber quality formation, contributing to the enhancement of cotton fiber quality.