The dissolution and release of active ions, such as Si4+, Al3+ and Ca2+, from fy ash directly afect the rate and extent of reaction product formation, which in turn afect the physical and mechanical properties of fy ash flling materials. In this study, low-calcium fy ash was soaked and activated in NaOH solutions with diferent concentrations (approximating the optimum dose range) for diferent lengths of time. The amounts of active ions leached and the changes in the mineral composition, chemical functional groups and surface morphology were tested and analyzed via ICP-OES, XRD, FTIR and SEM/EDS techniques. Based on these analyses, the reaction mechanism of alkali activation of low-calcium fy ash was further investigated. The results showed that the NaOH activation efect can signifcantly increase the amount of active ions leached from low-calcium fy ash. Notably, the amount of Si4+ and Al3+ leached clearly increased with increases in both NaOH concentration and soaking time. The plausible reaction mechanism is discussed in detail, which is that the alkali activator principally afected the surface of the vitreous particles of lowcalcium fy ash and induced difering surface modifcations in the dissolution stage, depolymerization stage, polycondensation and polymer gel stage and difusion stage. It was observed that the progress of the reaction is controlled by dissolution in the early stages, whereas activation is governed by difusion when the surfaces of the fy ash particles are covered by precipitates.