Abstract: The issue of water surface evaporation loss in river reaches caused by hydropower station construction is a significant social concern; existing research often focuses on changes in pan evaporation within reservoir areas but lacks in-depth accounting and comparative analysis of the actual water resource loss through evaporation. Targeting the quantitative analysis of reservoir surface evaporation before and after the operation of the Jinsha River A Hydropower Station, this study utilized Sentinel-2 and Landsat 8 satellite imagery as data sources to design a water area extraction method based on feature field screening and image reconstruction. Combined with local area fitting from single images, the time series of water area in the reach from 2018 to 2023 increased from the original 198 points to 294 points, achieving high-temporal-frequency monitoring of water area changes. The Penpan model after fitting the wind speed function, was employed to interpolate and extend the pan evaporation time series. Water surface evaporation rates were calculated by combining this with reconstructed conversion coefficients based on variations in water surface vapor pressure deficit. Integrating both reconstructed area and rate enabled evaporative water loss estimation. The results indicate that: ① After the hydropower station began operation, the substantial expansion of the water area in the reach coincided with a 25.5% increase in the daily average water surface evaporation rate compared to pre-reservoir conditions; the combination of these factors led to a 5.26-fold increase in daily evaporation water loss; ② The reservoir formed by the hydropower station altered the local microclimate, causing a "dual decrease" in both the evaporation radiation term and the aerodynamic term during December-June, resulting in reduced pan evaporation; conversely, a significant increase in the aerodynamic term during July-November caused pan evaporation to increase; ③ The superposition of pan evaporation and changes in wind speed and saturation deficit induced by the reservoir's thermal environment effect led to a maximum decrease of 55.17% in the reach's water surface evaporation rate during April-June compared to pre-reservoir levels, while rates increased substantially in other months, reaching a maximum increase of 90.31%. The research findings provide insights into understanding changes in reach water surface evaporation under the influence of reservoir impoundment and offer data support for the refined water resource management of the A Hydropower Station.