Remote sensing-based ecological index (RSEI) is a widely used index for monitoring eco-environmental quality (EEQ). However, the traditional estimations usually face poor stability in tracking the continuous EEQ changes. To overcome this issue, this study introduces an improved framework for RSEI estimation to enhance the temporal comparability of EEQ assessments. It incorporates annual mean values and temporal normalization of key ecological indicators to address limitations, including susceptibility to seasonal variability and reliance on single-phase data. Through applying to Liangshan Prefecture, a mountainous region in Southwest China, the improved RSEI captures both long-term trends and short-term ecological dynamics from 2001 to 2020, revealing relatively stable EEQ, with mean values ranging from 0.465-0.470. Regions experiencing significant improvement or degradation accounted for less than 10% of the total area. Environmental improvements were largely attributed to human disturbance reduction and ecological restoration projects in high-altitude areas, while degraded regions were concentrated in the Anning River Valley, driven by population growth and urban expansion, affecting 51% of the area. The assessments demonstrate that the improved method shows greater stability and spatial coherence than the traditional ones, providing a robust metric for tracking EEQ trends for environmental monitoring and sustainable development planning.