Water scarcity and ecological fragility have become prominent constraints on sustainable development in arid inland river basins. Evaluating and forecasting the water resources carrying capacity (WRCC) is critical for achieving a balance between socioeconomic development and ecological conservation. This study constructs a comprehensive early warning framework for WRCC in the Shule River Basin, Northwest China, integrating water resources, ecology, economy, and society into a multidimensional indicator system. A combined weighting method based on entropy weight and CRITIC was employed to objectively determine indicator weights. The TOPSIS model was used to assess WRCC levels from 2000 to 2022. To provide dynamic predictions, a system dynamics (SD) model was developed in Vensim, simulating WRCC evolution under four scenarios: status quo, economic priority, environmental protection, and integrated coordination. Control charts were used to establish early warning thresholds and classify WRCC levels. The results indicate that WRCC showed a U-shaped trend from 2000 to 2022, only decreasing in 2009 and then returning to a high load state by 2022. Obstacle factor analysis identified key constraints in sewage treatment capacity, ecological water use, and economic water-use efficiency. Forecasts show that the integrated and coordinated scenario produces the most favorable outcome, with WRCC reaching 0.627 b y 2035, demonstrating a balance between economic growth and ecological protection. The proposed early warning and simulation framework offers a practical and adaptable approach for WRCC assessment and management in arid regions. The results provide a scientific basis for regional water resource planning and policy formulation under the pressures of climate change and rapid development.