The effective and quantitative monitoring of spatial-temporal changes in eco-environment quality is critical for urban sustainable development and policymaking. In this study, four ecological factors (greenness, wetness, heat, and dryness) of Beijing were obtained by integrating MODIS imagery in each year from 2001 to 2020. Then, a remote sensing ecological index (RSEI), which is employed to assess the eco-environment quality, based on the four factors was constructed by using the Google Earth Engine (GEE) platform. Finally, the spatial-temporal changes in RSEI over the 20 years were analyzed. The results show that the eco-environment quality of Beijing improved from 2001 to 2020, with the average RSEI increasing from 0.586 in 2001 to 0.64 in 2020. The eco-environment quality in the mountainous areas of Beijing is higher than that in the urban areas. The area exhibiting deterioration decreased from 23.46% between 2001 and 2005 to 8.66% between 2015 and 2020, while the area with improved eco-environment quality increased from 13.04% between 2001 and 2005 to 21.64% between 2015 and 2020. We have thus assessed the spatial-temporal changes in eco-environment quality based on RSEI and GEE, which is critical to the investigation of the interactions between human activities and ecosystem services in Beijing.