In order to find out the effects of mild salt stress on photosynthetic physiology, growth index and yield of cotton under drip irrigation, six different salt stress (CK: 1.5 g/kg, T1: 3.0 g/kg, T2: 4.0 g/kg, T3: 5.3 g/kg, T4: 6.2 g/kg, T5: 7.3 g/kg) were applied in barrel cultivated cotton, and the changes in physiological growth and yield were studied to provide the theoretical basis for cultivation and management of cotton in salinity soil. The results showed that with the increase of salt stress and the extension of stress time, the photosynthetic physiological indexes of cotton showed a downward trend (P＜0.01). The effect of T1 and T2 on cotton net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), intercellular CO2 concentration (Ci) and leaf water use efficiency (WUE) were not significant (P＞0.05), while the effect of T3, T4, and T5 on photosynthetic physiological indexes showed significant inhibition (P＜0.05); Pn, Tr, Gs, and Ci in early growth under salt stress were decreased, and Ls value increased with increasing salt stress. Pn, Gs, and Ls in the opening stage were less than in the bud stage, and Ci was greater in the opening stage than in the bud stage. Plant height and leaf area in the early stage was significantly affected by salt stress, increased rapidly in the later stage in T2 and T3, and maximized in T3. Compared with CK, T1 and T2 did not affect yield, cotton boll number per plant, and boll quality, but the yield in T3 decreased rapidly. Through the sub salt tolerance function, the salt critical value of cotton in the whole growth period of 0—40 cm soil was 5.441 9 g/kg (St), and the salt tolerance limit was 44.201 6 g/kg (S0). Yield had a good fitting degree with Pn, WUE, plant height (H), and leaf area (S), the correlation coefficients were 0.69, 0.73, 0.50 and 0.72, respectively. In conclusion, mild salt stress led to the photosynthetic rate decline of cotton through stomatal limitation in growth stage, and through non stomatal limitation factors in late growth stage compared with moderate and severe salt stress. Photosynthetic parameters and salt stress degree were closely related, and cotton yield and photosynthetic indexes showed a good correlation.