Taking two cotton cultivars with different fiber strength (KC-1, average fiber strength 35 cN x tex(-1); and AC-33B, average fiber strength 32 cN x tex(-1)) as test materials, a field experiment with two planting dates (25 April and 25 May) was conducted in Nanjing of Jiangsu (lower reaches of Yangtze River Valley) and Xuzhou of Jiangsu (Yellow River Valley) to study the dynamic changes of plant sucrase, sucrose synthase, sucrose phosphate synthase, and beta-1,3-glucanase activities during the development of cotton fiber at different air temperatures and their relationships with fiber length and strength. In the development period of fiber elongation, sucrase and beta-1,3-glucanase activities were higher; while during the period of fiber thickening, sucrose synthase and sucrose phosphate synthase activities increased rapidly and were high, but sucrase and beta-1,3-glucanase activities had a rapid decrease. The higher sucrase activity in fiber elongation development period favored the fiber length development, while the rapid increase of sucrose synthase and sucrose phosphate synthase and the rapid decrease of sucrase and beta-1,3-glucanase activity favored the development of fiber strength. For cotton variety KC-1, the sucrase and beta-1,3-glucanase activities in its early development period and the sucrose synthase and sucrose phosphate synthase activities in its middle and later development periods were all higher than those of AC-33B. In this study, 23.3 degrees C was the optimal temperature for high strength fiber formation, and 23.3 degrees C - 25.5 degrees C was the optimal temperature for long length fiber formation.