Dryland ecosystems face increasing pressure from climate change, making understanding the carbon-water relationship crucial for sustainable development. This study investigates how this relationship, represented by the trade-off between water conservation (WC) and net primary productivity (NPP) (TWC-NPP), responds to aridity gradients in the Qilian Mountain Area (QLMA). Using data from 2000 to 2018, we employed a moving window method, generalized additive models, and XGBoost-SHAP to quantify the contributions of climate and vegetation cover to TWC-NPP variability. Our results showed that although both WC and NPP improved across the QLMA, TWC-NPP also increased, indicating a growing trade-off between these ecosystem services, especially in the eastern region. TWC-NPP responded nonlinearly to aridity and peaked at an aridity index (AI) of 0.39, located in the dry sub-humid zone near the humid boundary, indicating a dry-wet transition belt where carbon-water coupling shifts most strongly. Fractional vegetation cover (FVC) was identified as the dominant factor influencing TWC-NPP, surpassing the role of snowmelt under higher aridity conditions. Furthermore, the FVC threshold for influencing TWC-NPP decreased from 0.55 to 0.38 with increasing aridity, signifying heightened vegetation sensitivity under drier conditions. These findings highlight the crucial role of vegetation in mediating carbon-water trade-offs along aridity gradients and underscore the importance of considering these complex interactions when developing ecosystem management strategies in the QLMA under a changing climate.