Abstract: As global warming continued to intensify, the growth of vegetation was increasingly constrained by water availability. In recent years, ecological restoration initiatives in the Loess Plateau, such as "The Grain for Green project," led to significant improvements in the ecological environment of the region. However, the excessive restoration of vegetation resulted in the overconsumption of water resources and the degradation of vegetation, giving rise to new ecological issues, with the regional drought situation remaining severe. To investigate the current status and changes in water surplus and deficit index (WSDI) in the Loess Plateau and to seek effective solutions, this study defined WSDI as the ratio of effective precipitation to evapotranspiration. Utilizing remote sensing data and ground observations from 2001 to 2023, the research analyzed the temporal and spatial changes in temperature, precipitation, effective precipitation, evapotranspiration, normalized difference vegetation index (NDVI), and gross primary productivity (GPP) within the Loess Plateau. The goal was to clarify the state of water surplus and deficit in the region and to identify the key driving factors influencing changes in WSDI. The findings of the study revealed several critical points: (1) Over the past 23 years, the Loess Plateau exhibited a tight balance in WSDI on an annual scale, with a long-term average WSDI of 1.1. The average annual precipitation stood at 441 mm, while the evapotranspiration related to vegetation reached 353 mm. There were significant seasonal variations in WSDI, with winter experiencing the most severe water deficit, followed by spring. In contrast, summer and autumn exhibited a surplus of water. (2) Between 2001 and 2023, both effective precipitation and evapotranspiration showed an upward trend. However, the rate of increase in evapotranspiration surpassed that of effective precipitation, resulting in a downward trend in WSDI across various time scales. This decline was most pronounced in the summer months. Spatially, 91% of monitoring stations displayed a declining trend in WSDI at the annual scale, especially during the summer. (3) Different types of vegetation exhibited varying changes in WSDI. Grasslands showed the fastest decline in WSDI during the spring, while shrublands experienced the most significant changes in summer. Changes in the autumn and winter seasons were relatively minor, although both shrublands and croplands exhibited noticeable declines. (4) The relationship between temperature, precipitation, and vegetation affecting WSDI was complex and nonlinear. Changes in precipitation had both direct and indirect effects on WSDI in the Loess Plateau, with the greatest contribution coming from precipitation variability. Additionally, increases in summer vegetation productivity significantly influenced changes in WSDI. This research offered vital insights for managing farmland water resources, restoring vegetation, and optimizing the allocation of water and soil resources in the Loess Plateau, particularly in the face of future climate change challenges. By understanding the intricate dynamics of water surplus and deficit, policymakers and environmental managers could make informed decisions to promote sustainable ecological practices in the region.