Flowering time is an important ecological trait that determines the transition from vegetative to reproductive growth. Flowering time in cotton is controlled by short-day photoperiods, with strict photoperiod sensitivity. As the CO-FT (CONSTANS-FLOWER LOCUS T) module regulates photoperiodic flowering in several plants, we selected eight CONSTANS genes (COL) in group I to detect their expression patterns in long-day and short-day conditions. Further, we individually cloned and sequenced their homologs from 25 different cotton accessions and one outgroup. Finally, we studied their structures, phylogenetic relationship, and molecular evolution in both coding region and three characteristic domains. All the eight COLs in group I show diurnal expression. In the orthologous and homeologous loci, each gene structure in different cotton species is highly conserved, while length variation has occurred due to insertions/deletions in intron and/or exon regions. Six genes, COL2 to COL5, COL7 and COL8, exhibit higher nucleotide diversity in the D-subgenome than in the A-subgenome. The Ks values of 98.37% in all allotetraploid cotton species examined were higher in the A-D and At-Dt comparison than in the A-At and D-Dt comparisons, and the Pearson's correlation coefficient (r) of Ks between A vs. D and At vs. Dt also showed positive, high correlations, with a correlation coefficient of at least 0.797. The nucleotide polymorphism in wild species is significantly higher compared to G. hirsutum and G. barbadense, indicating a genetic bottleneck associated with the domesticated cotton species. Three characteristic domains in eight COLs exhibit different evolutionary rates, with the CCT domain highly conserved, while the B-box and Var domain much more variable in allotetraploid species. Taken together, COL1, COL2 and COL8 endured greater selective pressures during the domestication process. The study improves our understanding of the domestication-related genes/traits during cotton evolutionary process.