The evaluation of ecological environment quality and its changes is the key basis for ecosystem management, and also an important indicator to measure the effectiveness of regional ecological civilization construction. The coexistence of artificial oases, dry farmland, desert grassland, and flowing inland in the desert grassland belt of Ningxia, along with the Yellow River and its coastal wetlands, presents a relatively complete landscape encompassing mountains, water bodies, forests, fields, lakes, grasslands, and sands. This area serves as a typical location for studying the evaluation of ecological environment quality within multiple ecosystems coupling. Based on the Google Earth Engine (GEE) cloud platform, this paper optimizes the reconstruction of Landsat surface reflectance images from 2000 to 2020 for the desert steppe zone of Ningxia. It calculates four component indicators of regional wetness (WET), greenness (NDVI), dryness (NDBSI), and heat (LST), and estimates the regional ecosystem service value (ESV) using land classification data. By applying principal component analysis to weight the coupling of these indicators, an improved remote sensing-based ecological index (IRSEI) is developed, which monitors the ecological condition of the study area over a long time scale, and identifies the main driving factors influencing IRSEI. The results showed that: (1) The first two principal axes accounted for 84.66% of the total variance, with NDVI and WET being the main contributors to PC1; ESV was the main contributor to PC2. The inclusion of ESV in the evaluation analysis of IRSEI decreased the clustering of component indicators in PC1, and enhanced the diversity of IRSEI by spreading the component indicators. (2) The ESV in the study area showed a steady increasing trend, with the order of regulating service>support service>supply service>cultural service. Spatially, the value was higher in the northeast and lower in the southwest. (3) The mean value of IRSEI index ranged from 0.546 to 0.598 from 2000 to 2020, showing a unimodal fluctuating increasing trend, with a growth rate of 7.42%. In the past 20 years, the area change shifted from mainly medium and high levels to mainly very high and high levels, with the high level area increasing by 84.79%. (4) Precipitation was the main driving factor affecting the variation of IRSEI, and the overall driving factors were ranked as: precipitation > population density > temperature > land use intensity > gross domestic product. Altitude affects the IRSEI index indirectly by influencing climatic factors. This study objectively assessed the spatiotemporal patterns and drivers of regional ecological environment quality in the past 20 years, and offered valuable insights for guiding regional ecological restoration and sustainable development.