Abstract: The shortage of water resources and over-exploitation of groundwater are the primary factors limiting the sustainable development of agriculture in North China. In order to address the critical challenge of achieving a balance between "Grain Yield Stabilization and Groundwater Extraction Reduction," this investigation focused on the Shijin Irrigation District as a representative case study. A comprehensive, process-based simulation model of the district's integrated water cycle was developed by strategically coupling three established models: SWAP, MODFLOW, and WEAP. This integrated framework effectively captures the dynamic interactions between surface water, soil water, and groundwater. Four scenarios were designed for comparative analysis: the Current Scenario (S0), the Water-saving Irrigation Scenario (S1), the Planting Structure Adjustment Scenario (S2), and the Comprehensive Regulation Scenario (S3). Using these scenarios, long-term simulations from 2025 to 2036 of groundwater levels and crop yields were performed, and informed the formulation of an optimized multi-source water allocation strategy. The results indicated that the model demonstrates reliable performance in simulating groundwater level, soil moisture content, and crop yield, with determination coefficients (R²) exceeding 0.58. Compared with the Current Scenario (S0), the Water-saving Irrigation Scenario (S1) demonstrates significant benefits: the annual water saving of the irrigation district reached 72 million m³, the amount of groundwater extraction reduction reached 46 million m³, the groundwater level across the irrigation district raised cumulatively by approximately 3.5 m over the simulation period, and total grain yield fluctuated within the range of 1.056 billion ～1.126 billion kg. Under the Planting Structure Adjustment Scenario (S2), where the proportion of grain crops decreased from 70% to 60%, the groundwater level exhibited a slow upward trend, and the grain yield remained stable at over 1 billion kg. By systematically integrating water-saving irrigation techniques and adjustments in cropping patterns, this study proposed an optimized mode of multi-source water allocation suitable for the Shijin Irrigation District. This scheme prioritizes surface water supplied from the Gangnan and Huangbizhuang reservoirs, with groundwater serving as a supplementary source. Simulation results from 2025 to 2036 reveal that, compared to the current water allocation practice, the optimized mode raised the overall groundwater level of the irrigation district by about 4.68 m and the fluctuation range of total grain yield was only 34 million kg, achieving both groundwater extraction reduction and grain yield stabilization. The research results can provide effective modeling tools and a scientific basis for the modern management of water resources in the Shijin Irrigation District, as well as for water-saving transformation and control of groundwater over-exploitation in similar multi-water-source irrigation districts in North China.