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), we selected vermiculite, zeolite, and perlite as ameliorants. To conduct a comprehensive evaluation of the impact of filler ratios on hydraulic performance, contaminant adsorption, and operational life, a three-factor, three-level orthogonal experimental design was applied for the analysis, complemented by range analysis and analysis of variance. The findings demonstrated that the filler composition was the decisive factor governing the comprehensive performance of the system, with the optimal volumetric ratio of 3.75% zeolite, 3.75% perlite, and 1.25% vermiculite. Under this optimal ratio, a saturated hydraulic conductivity of 30.05 mm/h, a maximum adsorption capacity of 3054 mg/kg, and an estimated operational lifespan of 12.55 years were achieved. Range and variance analyses revealed that perlite primarily enhanced hydraulic conductivity and mass transfer efficiency by constructing a macroscopic pore structure, whereas zeolite and vermiculite contributed 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 \mathrmNH_4^+ -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 in bioretention systems. The findings can provide data support for the treatment of rainwater runoff carrying typical pollutant loads in loess regions of Northwest China.