To study the impact of continuous drought stress during the flowering and boll stage on cotton growth, as well as the quantitative expression and thresholds of soil drought, the cotton variety Xinluzao 78 was used as the experimental material. During the flowering and boll stage, different levels of drought treatments were designed (normal irrigation, and 90%, 70%, 50%, and 30% of the normal irrigation water amount), and the changes in cotton photosynthesis, agronomic traits, and soil relative water content under different treatments were measured. The results showed that compared with normal irrigation, soil relative water content, net photosynthetic rate, transpiration rate, leaf relative water content, plant height, and boll dry weight all decreased under different degrees of drought treatment, and the decrease was more significant as the duration of drought increased, ultimately leading to reduced yield. The photosynthesis and agronomic traits of cotton were correlated with changes in soil relative water content, showing a threshold response. By using a 5%, 10%, and 20% reduction in yield as the critical points, a linear correlation analysis was conducted between the drought degree D value, yield, and drought duration. The thresholds for mild drought, moderate drought, and severe drought were determined to be 0.253, 0.387, and 0.655, respectively, with higher levels of drought stress leading to earlier occurrence of drought thresholds. This study established the soil drought threshold indicators and their occurrence times under different levels of drought stress during the cotton flowering and boll stage, providing a reference for reasonable and timely deficit irrigation in cotton field production.
