River sludge is usually stabilized with cementitious binder and applied in pavement construction as sub-ground soil for sustainable treatment. However, the application of Ca-based binder, such as Portland cement and lime, is increasingly limited by the high energy consumption and CO2 emission. To solve this problem, granulated blast furnace slag (GBFS) and fly ash were used to synthesize binary-phases based geopolymer to stabilize sludge. Firstly, the mix proportion of geopolymer stabilizer was determined based on the influence of Si/Al and Na/Al molar ratio on the setting time and compressive strength of geopolymer. Subsequently, the effects of geopolymer content and curing age on the mechanical properties of stabilized sludge are studied, and the practical performance of stabilized sludge was evaluated with water stability, CBR, dry shrinkage and temperature shrinkage tests. Furthermore, the stabilization mechanism was revealed by the microstructure characterization with scanning electron microscopy and X-ray diffractometry. The experimental results show that the geopolymerization products include amorphous geopolymer gel, calcium silicate hydrate and calcium aluminate hydrate, which enhance the binding of soil particles and filled the voids, and thus improve the mechanical properties and road performance of stabilized sludge. It provides experimental foundation for the practical application of geopolymer stabilized sludge in pavement construction.