The Loess Plateau is located in the middle reaches of the Yellow River, with serious soil erosion and important ecological status. Scientific assessment of the ecological environment quality of the Loess Plateau is the premise to realize the sustainable development of regional ecology, which is of great significance to the improvement of the ecological quality of the Loess Plateau. Based on the Landsat series data, this study constructed the remote sensing ecological index (RSEI) and evaluated the ecological environment quality of the Loess Plateau. The results show that: (1) From 2006 to 2020, the average RSEI experienced three stages of degradation-slow improvement-rapid improvement; from 2005 to 2010, the RSEI decreased from 0.6465 to 0.5516, and the area of variation accounted for 46.7% of the total area. From 2010, the ecological environment quality began to improve. In 2015, the improved area accounted for 31.84%. By 2020, the improved area accounted for 45.6%. (2) On the whole, from 2006 to 2020, the ecological environment quality level of Ganquan County has improved, of which the improved area accounts for 33.4%, while the deteriorated area accounts for 14.74%, and the improved area is 2.3 times the deteriorated area. (3) From the perspective of spatial aggregation, there is aggregation in the distribution of ecological environment quality in Ganquan County, mainly H-H and L-L aggregation. The H-H region is located in the mountain forest area with relatively high altitude around Ganquan County, and the L-L region is mainly distributed from northwest to southeast near the Yanluo River and its tributaries, that is, the sandy hilly and gully area of the Loess Plateau. However, with the passage of time, the area corresponding to L-L gradually decreased, and its significance gradually decreased. In this study, RSEI ecological index was used to provide a scientific basis for timely and accurate determination of the spatial and temporal changes of ecological environment quality and economic development in the Loess Plateau.