Due to the limited local electricity consumption in northeastern, northwestern, and northern China, developers are committed to unlocking wind resource-rich areas in the mountainous regions of southern China, leading to the southward movement of wind power. Construction processes destroy vegetation (especially forests) and may cause subsequent ecological impacts. However, due to the scarcity of cloudless remote sensing images in southern China, the ecosystem impact of low-speed wind farm construction in southern China remains unclear, which is not conducive to decision-making. This study used the flexible spatiotemporal data fusion (FSDAF) model to fuse the normalized difference vegetation index (NDVI) value obtained by Landsat 5 satellite Thematic Mapper (TM)/Landsat 8 satellite Operational Land Imager (OLI) and the NDVI value obtained by Moderate-resolution Imaging Spectroradiometer (MODIS) images. Cloudless and high-resolution (30 m) fusion results were used to establish a system framework for evaluating and quantifying the impact of low-speed mountain wind farms on vegetation and soil erosion. The results showed that wind farm construction led to a significant decline in mountain vegetation growth. The impact ranges of wind farm roads were mainly within the 0–30 m buffer zone (the reduction range of NDVI was 0.04–0.22), and the impact ranges of wind turbine foundations were mainly within the 0–60 m buffer zone (the reduction range of NDVI was 0.00–0.26). In addition, the NDVI in regions damaged by mountain wind farm construction was more sensitive to climate change. After vegetation damage, the synergistic effect of factors such as rain and terrain may lead to the doubling of soil erosion (compared to 2010, the increase in soil erosion could reach 1195.96%). In the study area, roads should be scientifically planned to reduce their length, and vegetation restoration and environmental supervision should be strengthened.