The Qingtongxia Irrigation District in Ningxia is an important hydrological and ecological region. To assess its ecological environment quality from 2001 to 2021 across multiple scales and identify driving factors, a modified remote sensing ecological index (MRSEI) was developed by incorporating evapotranspiration. Spatial and temporal patterns were analyzed using the coefficient of variation, spatial autocorrelation, and semi-variogram methods, while influencing factors were explored via the optimal parameter geographical detector model. The MRSEI's first principal component loadings and rankings aligned with those of RSEI (average contribution: 81.31%), effectively reflecting spatiotemporal variations. At sub-irrigation district and landscape scales, ecological quality was slightly lower than at the district level but remained stable. Moderate and good ecological grades accounted for 36.28% and 33.38% of the area, respectively, at the district scale, and the moderate grade reached 70.48% on smaller scales. Spatial heterogeneity intensified with decreasing scale, and human activity lost explanatory power below a 5 km range. Human factors mainly drove ecological differentiation at the district scale, while natural factors dominated at finer scales. The MRSEI offers a novel tool for ecological assessment in arid/semi-arid areas and supports scale-adapted ecological protection strategies.