To determine how high temperature (HT) impairs cotton fiber elongation, greenhouse experiments compared two temperature regimes (CT, 28 ℃; HT, 38 ℃) for 12 d. The findings indicated that compared with CT, the fiber elongation rate initially increased during HT (0–8 d), but subsequently decreased, resulting in a significant decrease in cotton fiber length at harvest. At 3 and 6 d under HT, cell turgor pressure increased; additionally, the auxin and ethylene content in fiber significantly increased, promoting cell wall loosening and increasing elongation rate through regulating xyloglucan endoglycosyltransferases/hydrolases and expansin. At 12 d under HT, cellulose and hemicellulose decomposition were inhibited in fiber, which hindered cell wall loosening and restricted fiber elongation. Meanwhile, the lipid and callose content in fiber was increased, and the enhanced abscisic acid and H₂O₂ content promoted lignin synthesis by up-regulating the expression of WLIM1a gene. Both changes accelerated the initiation of secondary wall synthesis. Consequently, the transition stage that coincides with fiber elongation and secondary wall thickening was reduced by 0.3–0.5 d, leading to a decrease in fiber length. In summary, fiber length was reduced under HT, accompanied by restricted cell wall loosening and accelerated secondary wall synthesis.