Abstract: The study showed that under field condition, the activities of Sucrose Phosphate Synthase(SPS) appeared double peaks at blooming day and 18 days post anthesis(DAP) in leaves and cotton fibres of Shannong Brown 02, Fengkang 6 and Zhongmiansuo 45; the first peak surpassed the second peak and it decreased as a whole. And simultaneously soluble sugar content, sucrose content and photosynthesis rate had the same changes. The values of SPS activity, soluble sugar content, sucrose content and photosynthesis rate were the highest in Fengkang 6, second in Zhongmiansuo 45, third in Shannong Brown 02. The three cotton cultivars differ very significantly (P<1%) in corresponding observation index.