Abstract: The Loess Plateau, a critically important dryland agricultural region in northern China, faces significant challenges primarily due to the pronounced mismatch between the seasonal distribution of rainfall and the water requirements of its crops. To address these limitations, this study assessed the potential implementation zones, suitability levels, and yield improvement effects of rainwater harvesting and supplemental irrigation (RHSI) technology using integrated remote sensing and ground observation data from 2000 to 2018. Detailed spatiotemporal patterns of precipitation and evapotranspiration were rigorously analyzed to evaluate the fundamental feasibility of RHSI deployment across the plateau. The Soil and Water Assessment Tool (SWAT) was specifically employed to simulate the volume of harvestable rainfall runoff available for capture, while precise crop water requirements were calculated using the standardized Penman–Monteith equation. Subsequently, a sophisticated reinforcement learning–based optimization model was applied to achieve essential spatiotemporal synchronization between the available water supply (represented by simulated runoff) and the identified crop water demand (quantified as water deficits). This synchronization process was fundamental for accurately delineating potential RHSI implementation zones, determining the optimal application timing, and estimating the necessary supplemental irrigation volumes. The results clearly revealed a pronounced seasonal water imbalance across the Loess Plateau, characterized by an average regional water deficit of 40.32 mm occurring consistently from December through June, contrasted by a significant surplus averaging 45.15 mm accumulating from July through November annually. Areas exhibiting high potential for practical rainwater utilization were predominantly concentrated within the distinct hilly–gully region (D) and the eastern high-plateau areas located specifically east of the Liupan Mountains. The calculated annual crop water demand averaged 518 mm across the study region, displaying a discernible spatial gradient decreasing progressively from approximately 600 mm observed in the northwestern sectors down to about 400 mm measured in the southeastern areas. Crucially, 78.7% of this total annual crop water demand occurred during the vital spring and autumn growing seasons. The simulated average annual runoff depth reached 45 mm, with Zones D and F (the southern high-plateau subregion) collectively contributing a substantial 39% of the entire region's total runoff volume. Approximately 3.76 million hectares of existing rainfed farmland were conclusively identified as technically suitable for practical RHSI implementation, predominantly distributed across the key zones of D (hilly–gully), F (southern high-plateau), and E (rocky mountainous zones). The calculated average annual supplemental irrigation volume necessary per unit area reached 719 m³ ·hm⁻². This irrigation was typically applied an average of 3.45 times annually, with a single application event averaging 204.30 m³·hm⁻² in volume. Irrigation scheduling was highly concentrated during the critical April–June period specifically to address the peak crop water deficits occurring then. Clear regional differences in scheduling patterns emerged: for instance, 85% of irrigation events within the Ningxia irrigation zone (C) occurred during April–May, whereas in the southern hilly–gully subregion (D2), more than 85% of irrigation events were concentrated specifically in June. Analysis of interannual variability demonstrated that during wetter years, RHSI implementation could potentially expand to cover 4.04 million hectares, with a reduced average application volume of 142.50 m³ ·hm⁻² per event. Conversely, during drier years, the suitable area increased slightly to 4.41 million hectares, supported by a further reduced average application volume of 119.10 m³·hm⁻² per single irrigation. Simulated crop yield responses robustly confirmed that RHSI significantly enhanced agricultural productivity, with regional average yield increases ranging substantially from 30% to 80%. Within the core implementation areas, Zones D and F demonstrated particularly strong results, showing calculated average yield improvements of 49.16% and 41.83%, respectively. Maximum potential yield increases reached notably high levels: specifically, up to 97% for rapeseed and 95% for spring wheat, particularly achievable within the hilly–gully zones. Overall, this comprehensive assessment definitively demonstrates that RHSI constitutes an effective strategy for mitigating damaging seasonal water shortages and substantially.