The hydraulic characteristics of plant leaves can be reflected by leaf water potential and midrib traits. The correlation between the two factors can help to understand the balance between water supply and demand, which is significant for elucidating the ecological adaptation strategies of plants. Salix matsudana, family Salicaceae, is a deciduous tree with strong roots and wind resistance. As the main tree species used in northern China road shelterbelts, afforestation, and Shahuang saline wetland vegetation restoration, S. matsudana is widely distributed in the floodplains, lake embankment, channels, and roadsides, and plays an important role in ecological barrier construction. There were three main aims of this study: (1) to reveal the relationship between leaf water potential and midrib density, and between leaf water potential and midrib diameter in S. matsudana in different plots; (2) to analyze variation in the relationships along a moisture gradient; and (3) to identify the causes of the observed relationships. The study was conducted in early August of 2015 in a floodplain wetland near Heihe River in Zhangye City, Gansu Province, China (38°59'22.3" N, 100°24'33.9" E). Three sample plots, at intervals of 70 m, were set up along a soil moisture gradient ordinally from the area near the water body to the wetland edge, plot I (69.23%), spot Ⅱ (48.38%) and spot Ⅲ (35.27%). GPS was used to record the latitude, longitude, and altitude of each plot; community traits were investigated using a diagonal method, and all individuals of S. matsudana were used for measurements of height and canopy. At each plot, 5 individuals of S. matsudana at 4 vertices and diagonal intersections were selected for measurements of leaf water potential, midrib diameter, midrib density, chlorophyll, specific leaf area, leaf temperature (T_leaf), photosynthetically active radiation (PAR), net photosynthetic rate (Pn), and transpiration rate (Tr). The standardized major axis (SMA) estimation method was used to examine the covariation between leaf water potential and midrib traits. The main results were as follows. First, with the decrease of soil moisture, the midrib density, Pn, Tr, T_leaf, and PAR increased gradually (P < 0.05), while the height, canopy, density, midrib diameter, specific leaf area, leaf water potential, and chlorophyll content of S. matsudana decreased gradually (P < 0.05). Second, the relationship between leaf water potential and midrib traits varied with soil moisture: there was a highly significant negative correlation between leaf water potential and midrib density at spot Ⅰ and spot Ⅲ (P < 0.01), whereas the correlation reached a significant level at spot Ⅱ (P < 0.05). Furthermore, there was a significant negative correlation between leaf water potential and midrib diameter (P < 0.05). We can conclude that variations between leaf water potential and midrib traits of S. matsudana along a soil moisture gradient could reflect plant acclimation.