Abstract: To address the critical limitations of bioretention systems in the Loess region of Northwest China, specifically the poor matrix permeability, limited contaminant adsorption capacity, and inadequate long-term operational stability inherent to local soils, this study proposes a synergistic modification strategy utilizing functional fillers; based on a benchmark soil mixture (BSM) comprising 75% sand and 25% loess (v/v), vermiculite, zeolite, and perlite were selected as ameliorants, and a comprehensive evaluation of the impact of filler ratios on hydraulic performance, contaminant adsorption, and system longevity was conducted using a three-factor, three-level orthogonal experimental design, complemented by range analysis and analysis of variance. The findings demonstrate that the filler composition is the decisive factor governing the comprehensive performance of the system, with the optimal volumetric ratio determined to be 3.75% zeolite, 3.75% perlite, and 1.25% vermiculite; under this optimal ratio, a saturated hydraulic conductivity of 30.05 cm・h⁻¹, a maximum adsorption capacity of 3574.26 mg・kg⁻¹, and an estimated operational lifespan of 12.55 years were achieved. Statistical analyses revealed that perlite primarily enhances hydraulic conductivity and mass transfer efficiency by constructing a macroscopic pore structure, whereas zeolite and vermiculite contribute predominantly to chemical adsorption and interface microenvironment regulation, respectively; multi-scenario simulations further verified that, under different rainfall recurrence intervals, the optimized system exhibited significantly superior removal efficiencies for NH₄⁺-N, total nitrogen, total phosphorus, and COD, along with enhanced resistance to shock loads, compared to the traditional BSM system. This study confirms that the synergistic combination of zeolite, perlite, and vermiculite effectively overcomes the inherent limitations of loess-based substrates, achieving the dual optimization of hydrological regulation and water purification functions, and thereby providing a robust theoretical basis and quantitative design guideline for the precise construction of bioretention facilities in sponge city initiatives within the Loess region.