Improving agricultural production under tightening water constraints requires a clear understanding of long-term changes in crop water productivity (CWP). This study integrated remote sensing, Mann-Kendall trend analysis, and GeoDetector to characterize the spatiotemporal patterns and trends of yield and CWP for wheat, maize, and sunflower and to examine factors associated with CWP variability in the Hetao Irrigation District (Hetao), the largest irrigation district in the upper Yellow River basin, during 2000–2021. Results showed that wheat yield remained generally stable, while maize and sunflower yields increased, with district-wide slopes of at least 0.1500 and 0.0153 t ha⁻¹ yr⁻¹, respectively. The multi-year mean CWP values were 1.56, 2.36, and 0.97 kg m⁻³ for wheat, maize, and sunflower, respectively. Maize CWP increased markedly (slope ≥ 0.0383 kg m⁻³ yr⁻¹), reaching 3.44 kg m⁻³ in 2021, while sunflower CWP increased from 0.76 to 1.08 kg m⁻³ and wheat CWP remained largely stable. GeoDetector analysis showed that annual maximum Normalized Difference Vegetation Index (NDVI_max,a), as an indicator of crop growth status, and Normalized Difference Salinity Index (NDSI), as an indicator of surface salinity conditions, had the highest q values for CWP. Their interaction showed the strongest enhancement effect among the selected factor combinations, with Δq values of 0.232, 0.222, and 0.194 for wheat, maize, and sunflower, respectively. The explanatory power of NDSI increased significantly for wheat (slope = 0.0009, p < 0.05), whereas the q values of both NDVI_max,a and NDSI both decreased significantly for maize and sunflower (p < 0.05). Sub-district-scale analysis further revealed contrasting crop-specific trends, with wheat CWP decreasing most strongly in Jiefangzha and maize CWP increasing markedly in Wulate. These results reveal crop-specific and spatially heterogeneous responses of CWP to changing water–salinity conditions and support differentiated management of irrigation, salinity, and cropping structure in large arid irrigation districts.