This study unravels the hierarchical mechanisms through which grazing intensity modulates sheep production performance in desert grassland ecosystems, mediated by plant-forage selection, rumen fermentation dynamics, and microbial community restructuring. Using structural equation modeling (SEM) and 16S rRNA sequencing across three grazing intensities (light, LG; moderate, MG; heavy, HG), we demonstrate that Stipa breviflora dominance acts as a critical ecological pivot. Intensified grazing pressure selectively amplified dietary reliance on Stipa breviflora by 1.52-fold (+/- 0.09 SD), triggering a fermentation paradox. This included higher volatile fatty acid (VFA) yields, with acetate increasing by 19 % and the A/P ratio rising by 15 %. At the same time, pH dropped from 7.45 +/- 0.07 in the light grazing group to 7.19 +/- 0.07 in the heavy grazing group. Additionally, total weight gain decreased by 37 % compared to the light grazing group (p < 0.05). LEfSe analysis identified grazing-driven microbial guilds: LG-enriched Christensenellaceae (fermentation moderation), HG-dominant Lactobacillales (acidogenesis), and Succiniclasticum (branched-chain VFA production). Despite elevated alpha diversity under HG (Chao1 index increased by 22.6 %), functional redundancy buffered ecosystem performance (VFA-pH dynamics uncorrelated with diversity, p > 0.05). KEGG predictions revealed grazing-intensity-specific metabolic signatures: HG microbiomes prioritized non-ribosomal peptide synthesis (stress responses), while LG upregulated selenocompound metabolism (antioxidant defense). SEM confirmed tripartite cascading effects (CFI=1.0): grazing -> Stipa -> fermentation disorder -> growth suppression. Our findings expose a metabolic trap where high-output fermentation impairs host energy harvest-a critical disconnect from traditional carrying-capacity models. We established a scientific basis for optimized grazing management practices in desert steppe ecosystems that simultaneously enhance livestock health and production efficiency. This work redefines sustainable desert pastoralism by revealing how forage changes under different stocking rates, through a plant-microbe-livestock triad, steer microbial consortia to shape ecosystem outcomes, providing a mechanistic foundation for grazing management that ensures both livestock productivity and ecological resilience.