Abstract: [Objective]In response to geohazards such as slope instability and landslides caused by the strong water sensitivity and poor structure of loess in Northwest China, this study investigates loess improvement techniques to ensure the safety of engineering construction and long-term operation. [Methods]In this study, Malan loess from Northern Shaanxi was improved using a fly ash–polyacrylamide (PAM) composite. A systematic experimental program, including direct shear tests, falling-head permeability tests, and scanning electron microscopy (SEM) analysis, was conducted to investigate the effects of different admixture ratios on the shear strength and permeability of the improved loess and to reveal the underlying microstructural mechanisms. [Results]THE RESULTS DEMONSTRATE THAT THE FLY ASH-PAM COMPOSITE SIGNIFICANTLY IMPROVES BOTH THE SHEAR STRENGTH AND IMPERMEABILITY OF THE LOESS. THE OPTIMAL MIX FOR SHEAR STRENGTH WAS FOUND TO BE 10% FLY ASH AND 0.3% PAM, WHICH INCREASED THE COHESION BY 72.2% COMPARED TO UNTREATED LOESS. FOR PERMEABILITY, A MIX OF 5% FLY ASH AND 0.5% PAM ACHIEVED A SUBSTANTIAL REDUCTION IN THE PERMEABILITY COEFFICIENT BY 91.7%. Microstructural analysis reveals that fly ash exerts both physical pore-filling and rigid skeletal support effects, while also generating C-A-S-H gels and ettringite crystals through pozzolanic reactions, which fill voids and cement soil particles. Meanwhile, the long-chain molecules of PAM form a flexible network via an “adsorption–bridging–cementation” mechanism, transforming the soil structure from a disordered arrangement of individual particles into an ordered framework of interconnected aggregates. The synergistic interaction between these components significantly enhances soil compactness and structural integrity, leading to concurrent improvements in mechanical properties and impermeability. [Conclusion]THE FLY ASH-PAM COMPOSITE TECHNIQUE PRESENTS AN ECONOMICAL AND ENVIRONMENTALLY FRIENDLY APPROACH FOR LOESS IMPROVEMENT. THE CLARIFIED SYNERGISTIC ENHANCEMENT MECHANISM PROVIDES A THEORETICAL FOUNDATION AND PRACTICAL REFERENCE FOR ENGINEERING CONSTRUCTION AND DISASTER PREVENTION IN LOESS REGIONS, UNDERSCORING THE METHOD'S SIGNIFICANT THEORETICAL AND PRACTICAL VALUE.