A cotton molecular breeding method includes (1) obtaining a chr.7 single QTL segment substitution line with high fiber strength, and (2) using the obtained chr.7 single QTL segment substitution line as a non-recurrent parent () to cross with a recurrent parent (♀) to obtain an F1 generation. Then (3) the F1 generation is back-crossed with the recurrent parent to obtain a BC1F1 generation, and (4) plants of the BC1F1 generation are screened for plants carrying the chr.7 single QTL segment, and then the screened plants are back-crossed with the recurrent parent for 2-4 times to obtain a BCnF1 generation. The method also includes (5) screening plants of the BCnF1 generation for plants carrying the chr.7 single QTL segment, and selfing the screened plants for 1-3 times to obtain a bred line BCnFm having the high cotton fiber strength.