Monitoring vegetation dynamics is fundamental to assessing the conservation efficacy of large-scale protected areas, particularly for flagship species like the giant panda. However, the Giant Panda National Park (GPNP) faces a unique challenge: it is geographically bisected by the Hu-Line, a profound demographic and climatic divide in China. This study integrates MODIS Enhanced Vegetation Index (EVI) time-series data (2000–2018) with redundancy analysis (RDA) to quantify spatiotemporal vegetation patterns and disentangle the driving forces of the Hu-Line, climatic shifts, anthropogenic interventions, and protected-area status and long-term conservation/restoration measures in areas later consolidated into the GPNP (the GPNP was formally established in January 2017). Our results reveal that the Hu-Line functions as a rigid biophysical barrier rather than merely a demographic boundary. Vegetation vigor in the southeastern sector was significantly higher than in the northwestern sector (p < 0.01), a disparity primarily driven by climatic constraints which explained 32.75% of the total variation. We identified a warming-greening mechanism where rising temperatures promoted vegetation growth in this high-altitude ecosystem. Crucially, while the GPNP exhibited a consistent recovery trend post-2013 within the current park boundary, a concerning leakage effect was detected in the contiguous non-protected areas (NGPNP), where 30.12% of the land experienced vegetation decline. These findings suggest that strict protection within the core protected areas may have displaced pressure to the fragile periphery. We conclude that effective management requires spatially differentiated strategies respecting the Hu-Line's natural limits, and urgently recommend integrating the contiguous zones into an Other Effective Area-based Conservation Measures network to ensure landscape-level connectivity and buffer the core habitat.