Abstract: The North China Plain is one of the most important grain-producing regions in China, characterized by an intensive double-cropping system of winter wheat and summer maize, and plays a vital role in food security. However, long-term intensive agricultural production, combined with limited surface water availability, has led to severe groundwater overexploitation and increased pressure on the regional groundwater quality. Quantifying the environmental costs associated with grain production is essential for improving agricultural water management and promoting sustainable development in the North China Plain. In this study, the Ziya River Plain, a representative high-yield and groundwater-overexploited area in the North China Plain, was selected as the study area. The assessment of grain production, agricultural water consumption, and nitrogen leaching was conducted using the Soil and Water Assessment Tool (SWAT) model. The SWAT model was calibrated and validated using long-term observed data of key variables related to water, nitrogen, and crop growth, including evapotranspiration, leaf area index, crop yields of winter wheat and summer maize, and nitrate leaching. Based on the validated model, long-term simulation was performed to quantify water and nitrogen processes and grain yield dynamics from 1980 to 2022. Model performance evaluation results indicate that the SWAT model can effectively reproduce the regional water and nitrogen processes and crop growth dynamics. The coefficients of determination (R²) and Nash–Sutcliffe efficiency coefficients for simulated evapotranspiration and leaf area index were greater than 0.60. For winter wheat and summer maize yields, the mean relative error and normalized root-mean-square error were both less than 0.25, and the agreement index was higher than 0.60, demonstrating satisfactory model performance. Simulation results show that cumulative grain production in the Ziya River Plain reached 533.4 million t during the period 1980–2022. This high level of grain output was supported by a total groundwater consumption of 95.3 billion m³, indicating a strong dependence of regional agricultural production on groundwater resources. Meanwhile, total nitrogen leaching amounted to 4.9 million tons, with an average nitrogen leaching intensity of 109.6 kg(N)/hm², reflecting substantial nitrogen losses to the subsurface environment under long-term intensive fertilization application. The results reveal pronounced trade-offs among grain production, groundwater depletion, and nitrogen leaching in the study area. Overall, this study provides a comprehensive, process-based assessment of the environmental costs of grain production in a typical groundwater-overexploited agricultural region of the North China Plain. The findings offer important implications for agricultural water management, particularly for optimizing irrigation and nitrogen management strategies, mitigating groundwater depletion and non-point source pollution, and supporting the coordinated achievement of food security, water security, and ecological environmental sustainability.