Optimizing water and nitrogen management in cotton fields in northwest China can increase cotton yield by improving leaf number and canopy light environment. By testing different irrigation patterns and nitrogen fertilizer rates, researchers found that under certain conditions, a specific nitrogen rate can lead to higher seed cotton yield. The number of leaves in the middle canopy plays a crucial role in photosynthetic response to nitrogen and water supply. Leaf number is key in regulating canopy leaf inclination and light interception, impacting overall cotton yield. This study suggests that considering both leaf number and light energy utilization is important for maximizing cotton production.