<p>Ecologically fragile areas globally face the critical challenge of balancing ecological restoration with sustainable development, including low-carbon transitions. Understanding the synergistic mechanisms between ecological security and carbon sink (CS) is essential but remains underexplored, particularly regarding how spatial optimization strategies can enhance both. This research selects Huan County, a representative ecologically fragile area within the Loess Plateau, as a case study. It establishes a comprehensive framework for "systemic understanding, process analysis, and spatial optimization" and integrates spatial analysis models to examine the spatio-temporal dynamics and synergetic mechanisms between ecological security patterns (ESPs) and CS. The study reveals a nonlinear synergy where ESPs' spatial configuration significantly regulates CS capacity. The ecological network connectivity in Huan County has been declining from 2000–2020, leading to a heterogeneous spatial pattern characterized by stronger connectivity in the eastern and western regions compared to the central region, which shows high spatial coupling with the differentiation of CS capacity. Through multi-scenario spatial partition optimization simulations, the addition of 17 ecological corridors and 77 ecological stepping stones improved ecological network connectivity, resulting in an 8.92 × 10⁶ t increase in total CS capacity. This research establishes a replicable spatial optimization framework that facilitates the synergistic integration of ecological conservation and CS enhancement in similar ecologically vulnerable regions, effectively addressing the gap in practical approaches for co-benefits.</p>