The Hetao Irrigation District is an important arid-oasis agricultural system in the Yellow River Basin, where water scarcity has become a major constraint on high-quality regional development and irrigation water use dominates the district’s water-use structure. Irrigation water resources carrying capacity represents the application of the broader concept of water resources carrying capacity to irrigation systems. Existing studies have largely overlooked long-term assessments of irrigation water resources carrying capacity and the identification of its dominant patterns of variation at the irrigation-district scale. This study aimed to evaluate irrigation water resources carrying capacity in the Hetao Irrigation District from 2001 to 2023 and identify its dominant patterns of variation. An evaluation indicator system was developed, and a TOPSIS model incorporating combined subjective–objective weighting was used to assess long-term changes in carrying status, while principal component analysis was applied to identify the dominant patterns of variation. The results showed that improvement in the irrigation water-use efficiency subsystem partially offset the adverse effects of the decline in the water availability subsystem, resulting in an overall improvement in carrying status. Linhe District ranked highest among the county-level units, whereas Dengkou County ranked lowest. The variation in carrying capacity was summarized into three dimensions: joint variation in water availability and water-use scale; a mixed pattern linking irrigation water-use intensity, the degree of water resource utilization, and groundwater conditions; and variation in effective irrigation coverage. By integrating long-term carrying-status assessment with the identification of dominant patterns of variation, this study reveals a compensatory relationship between improved irrigation water-use efficiency and declining water availability, thereby providing a new perspective on carrying-status evolution at the irrigation-district scale.