This paper investigated the influence of activator concentration, reaction time and fineness of fly ash on the dissolving character of the active Si/Al in fly ash activated by alkali. The results indicated that with the increase of the NaOH solution concentration, the dissolving percentage of active Si/Al in the fly ash went up. The finer the fly ash particles were, the higher dissolving percentage of active Si/Al was obtained. Under strong basic (2M NaOH solution) and boiling condition, the reaction time of active Si/Al dissolving was short and less than 10 minutes. The content of vitreous phase contributed to the content of active components. Active Si existed in the state of Q0 unit which presented an isolated silicon-oxide tetrahedron in fly ash, and active Al exhibited as Al[OSi]3 and Al[OSi]4 units in 4 coordinate. As the fly ash was activated by alkali, smooth shell of fly ash was gradually corroded by NaOH solution, glass phase was dissolved and crystal phase was exposed to the outside.