Abstract: Background Long-distance oil and gas pipelines are typical linear engineering projects that generate strong, corridor-shaped disturbances to surrounding ecosystems, particularly in mid-subtropical hilly regions characterized by fragile ecological backgrounds. Quantitatively identifying the spatiotemporal evolution of ecological quality and its spatial aggregation patterns during different project stages is essential for understanding the ecological effects of pipeline construction and for supporting targeted ecological restoration and management. Taking the Weiyuan–Leshan natural gas pipeline project as a case study, this research aims to evaluate the impacts of pipeline construction and operation on ecological quality and to clarify the spatial differentiation mechanisms of ecological degradation and recovery along the pipeline corridor.Methods Landsat multispectral images acquired in 2020 (pre-construction), 2022 (construction stage), and 2024 (operation stage) were selected. Four ecological indicators were extracted, including soil-adjusted vegetation index (SAVI), tasseled cap wetness (WET), land surface temperature (LST), and normalized difference built-up and bare soil index (NDBSI). After standardization, principal component analysis (PCA) was applied to integrate these indicators and construct the Remote Sensing Ecological Index (RSEI) as a comprehensive measure of ecological quality. Spatiotemporal changes in ecological quality were quantified using RSEI differences (ΔRSEI) between stages. The Getis–Ord Gi* statistic was employed to identify hotspots and coldspots of ecological change and to characterize spatial clustering patterns associated with pipeline construction. In addition, distance-based analysis was conducted to examine the attenuation of ecological disturbance intensity away from the pipeline axis. Results The RSEI effectively integrates information from SAVI, WET, LST, and NDBSI and reliably characterizes the comprehensive ecological response to pipeline construction. The mean RSEI value decreases from 0.7728 in 2020 to 0.7019 in 2022 and then increases to 0.7577 in 2024, indicating significant ecological degradation during the construction stage followed by partial recovery during operation. SAVI and WET show a decreasing–increasing trend, while NDBSI and LST exhibit an increasing–decreasing trend. The area classified as “excellent” ecological quality declines from 9,445.52 hm² in 2020 to 6,272.54 hm² in 2022 and recovers to 7,942.26 hm² in 2024. Over the entire study period, the cumulative degraded area reaches 1,189.33 hm², whereas the improved area amounts to 224.70 hm². Hotspot–coldspot analysis reveals strip-shaped coldspot clusters along the pipeline corridor during the construction stage, while hotspots become more fragmented during the operation stage. A persistent “north-cold and south-hot” spatial pattern is observed across the full study period. Ecological disturbance intensity exhibits a clear distance-decay effect, with the strongest impacts occurring within 0–50 m of the pipeline and diminishing beyond approximately 200 m.Conclusions Pipeline construction exerts significant and spatially concentrated negative impacts on ecological quality during the construction stage, while ecological conditions partially recover during the operation stage but do not fully return to pre-construction levels. The spatiotemporal differentiation and hotspot/coldspot patterns of ecological quality are jointly controlled by ecological background conditions, construction disturbance intensity, and the effectiveness of post-construction restoration measures. The integrated RSEI-based assessment framework combined with spatial hotspot analysis provides an effective approach for identifying key ecological impact zones of linear engineering projects and offers scientific support for ecological protection and soil and water conservation management in mid-subtropical hilly regions.