Hua Jialing represents a typical ecological barrier zone on the Loess Plateau, where its forest-grassland ecosystem plays a pivotal role in regional ecological security and soil and water conservation. This study employs multi-source remote sensing and meteorological data from 2000 to 2024 to construct an Ecosystem Quality Index (EQI) integrating coverage, structure, function, water regulation, and ecological stress using the entropy weight method. It systematically assesses the spatiotemporal evolution of forest-grassland ecosystem quality and its climate-driven mechanisms. Methodologically, trend analysis (coefficient of variation, Mann-Kendall, Sen slope, Hurst index), interpretative machine learning (XGBoost-SHAP), and piecewise structural equation modelling (pSEM) were combined to reveal long-term changes, stability characteristics, and threshold effects within the forest-grassland ecosystem. Results indicate: (1) Forest-grassland EQI increased from 0.36 to 0.53 (approximately 45.68%), while forest-grassland coverage rose from 43.04% to 59.03%, demonstrating significant ecological restoration and engineering effectiveness. (2) Topographic effects show higher stability in low-altitude, gently sloping, and sunny-aspect areas, whereas high-altitude and steep-slope regions exhibit stronger resilience. (3) SOIL (threshold similar to 20 mm), PR (threshold similar to 500 mm), and SRAD (threshold similar to 1900-1980 W m-2) emerged as dominant factors. PR exhibited the strongest direct effect, SOIL exerted a significant negative impact, and SRAD's negative indirect effect surpassed its direct effect. (4) Spatial variations in ecosystem stability are pronounced. Future projections indicate that 35.77% of areas will continue to improve, yet 60.41% face degradation risks, necessitating targeted interventions and long-term monitoring. This study not only validates the long-term efficacy of ecological engineering but also uncovers threshold values and interactive mechanisms of key environmental factors, providing guidance for precision management and sustainable stewardship across the Loess Plateau and other ecologically fragile regions.