To systematically reveal the synergistic regulatory mechanisms of thinning intensity and water-nitrogen addition on leaf functional traits in plantations，this study conducted a split-plot experiment with Populus tomentosa plantations in the Yellow River Flood Plain. Two thinning intensities（50% and 75%） and three water-nitrogen treatments（water-nitrogen addition，single water addition，and control） were established. The response characteristics and coupling relationships were analyzed from three dimensions：leaf water status，structural construction，and nutrient stoichiometry. The results indicate that：（1）Leaf relative water content remained generally stable，whereas leaf dry mass water content exhibited high sensitivity to thinning and its interaction with water-nitrogen treatments，showing a significant increase of 43.05% under 75% thinning with water-nitrogen addition. （2）Structural traits showed no significant overall changes，with low variation in leaf dry matter content and high variation in specific leaf mass，demonstrating potential plasticity. （3）Among nutrient traits，leaf carbon content remained relatively stable，whereas leaf nitrogen and phosphorus contents as well as their stoichiometric ratios responded significantly to the treatments. The leaf N∶P ratio（mass ratio）ranged from 11.06 to 18.46，with an average value of 13.94，indicating that the P.tomentosa plantations in the study area were generally under nitrogen limitation. （4）Multivariate analysis revealed significant coupling relationships among leaf water content，structural traits，and nutrient allocation，with leaf dry mass water content and specific leaf mass identified as key traits driving nutrient partitioning. （5）Principal component analysis revealed that the first two principal components cumulatively explained 66.4% of the variation. Thinning at 75% promoted leaf adjustment towards high resource-use efficiency，while 50% thinning maintained a conservative strategy. Different water and nitrogen treatments primarily diverged along the nutrient structure dimension，with the single water addition treatment showing strong association with nutrient ratio traits. Overall，thinning and water-nitrogen regulation drive coordinated changes in leaf functional traits by altering resource availability，thereby influencing plant resource utilization strategies.