Since the commissioning of the Middle Route of the South-to-North Water Diversion Project, the groundwater level in the North China Plain has gradually recovered. The existing irrigation system has not fully considered the impact of groundwater on crop growth, which needs to be investigated. This paper used AquaCrop model to simulate the growth process of summer maize in Shijin Irrigation District, and explored the influence of shallow groundwater depths (set at 0.5, 1.0, 1.5, 1.8, 2.0, 2.5, 3.0, 3.5, 4.0 m) on soil moisture and crop yield. Subsequently, the optimal threshold of groundwater level was determined by the coupling coordination model, and the irrigation system was optimized by the threshold regulation for the first time. The results show that a shallow groundwater depth of 1.8-2.0 m is most suitable for maize growth. When the groundwater depth is less than 2.0 m, the water required for maize growth is primarily supplied by groundwater, eliminating the need for additional irrigation and saving approximately 285.6×106 m3 water across the entire district. As for the groundwater depth of 2.0-4.0 m, irrigation is required once during both seedling and jointing stages in dry years, only once during the seedling stage in normal years, and not at all in wet years, saving approximately 81.6×106 m3 water. The results can provide a technical support for the sustainable utilization of agricultural water resources in irrigation districts.