Loess, widely distributed across the Loess Plateau and possessing substantial engineering potential, requires effective stabilization due to its inherently loose structure, high silt content, and low natural cohesion. This study investigates fiber-binder stabilization of loess and develops a performance-driven optimization framework to enhance mechanical behavior and microstructural compactness while improving material efficiency. Laboratory experiments were conducted on mixtures incorporating cement, polyethylene fibers, and a polymer-based binder. Significant improvements in compressive strength and shear resistance, together with a reduction in average pore size, were observed. SEM and mercury intrusion porosimetry analyses revealed that cement hydration products form a continuous load-bearing skeleton, binder gels enhance interparticle bonding and pore densification, and fibers bridge microcracks and subdivide large pores, thereby refining the internal structure. Response surface modeling and global sensitivity analysis quantified nonlinear interactions among components, showing that fiber and binder contribute more substantially to performance variance once structural continuity is established. A multi-objective optimization framework integrating NSGA-II and entropy-AHP-weighted TOPSIS identified an optimal mixture with validation errors below 6.5%. This study provides a quantitative and interpretable pathway for performance-oriented and sustainable loess stabilization design.