Abstract: 【Objective】 To explore the response law and screen the photosynthetic indexes related to cold tolerance of cotton by analyzing the effects of different low temperature stress on photosynthetic characteristics of different cotton varieties in bud stage so as to provide a theoretical basis and technical guidance for identification of cold tolerant resources and breeding of cold tolerant cotton varieties. 【Methods】 Three low temperature gradients (T₁ (20/15℃, day / night), T₂ (15 / 10℃), T₃ (10 / 5℃) and CK (28/20℃) were applied in the bud stage. 【Result】 Net photosynthetic rate (PN), stomatal conductance (GS), transpiration rate (TR) and stomatal limit value (LS) of cotton in bud stage decreased significantly under low temperature stress, intercellular carbon dioxide concentration (CI) decreased first and then increased, and water use efficiency (WUE) increased first and then decreased. 【Conclusion】 The degree of low temperature stress can be judged according to the stomatal conductance (GS), transpiration rate (TR), intercellular carbon dioxide concentration (CI) and water use efficiency (WUE). Under T₁ stress, the decrease of photosynthetic rate of cotton in bud stage is caused by stomatal limiting factors; under T₂ stress, the decrease of photosynthetic rate of cotton in bud stage is mainly caused by stomatal factors first and then by non stomatal factors; under T₃ stress, non stomatal factors are the main factors limiting photosynthesis of cotton in bud stage. Low temperature stress inhibits photosynthesis of leaves in bud stage. The higher the stress intensity, the more obvious the inhibition on cotton growth in bud stage and the longer the duration of low temperature treatment, the more obvious the inhibition on cotton growth at bud stage.