Abstract: Elucidating the water-driven mechanisms underlying the formation of banded vegetation and identifying precipitation threshold effects are of critical importance for guiding ecological restoration and scientifically informed greening strategies in arid regions. In this study, the Shiyang River Basin and the endorheic region of the Junggar Basin were selected as contrasting study areas. Quantitative models were developed to characterize the relationships between the Normalized Difference Vegetation Index (NDVI), precipitation, and topographic factors, with the aim of revealing the regulatory role of water availability in shaping vegetation spatial patterns. The results show that: (1) Vegetation distribution is jointly regulated by precipitation and topography. NDVI is positively correlated with precipitation and negatively correlated with both elevation and slope, although the dominant controlling factors vary regionally. In the Shiyang River Basin, NDVI is primarily influenced by elevation and slope along the horizontal gradient, whereas precipitation exerts the dominant control along the vertical gradient. In contrast, vegetation NDVI in the Junggar Basin is jointly controlled by precipitation, elevation, and slope along the horizontal gradient. (2) Banded vegetation exhibits a distinct lower threshold of annual precipitation for sustained growth, estimated at approximately 150 mm in the Shiyang River Basin and 110 mm in the Junggar Basin. These findings demonstrate that the synergistic effects of precipitation and topographic factors constitute the key mechanism governing the formation and maintenance of banded vegetation in arid environments, providing a scientific basis for region-specific ecological restoration and optimized vegetation management.