The reaction processes of mineral admixtures in a cement-silica fume-fly ash ternary cementitious system aged for different time were quantitatively studied by means of selective dissolution and non-evaporative water content methods. To determine separately the degree of pozzolanic reaction for silica fume in the ternary system, fly ash was replaced by the inert quartz powder with the same fineness and same dosage in order to eliminate the dilution effect and heterogeneous nucleation effect. Then the reaction degree of fly ash can be calculated from the total reaction degree of all the admixtures in the system. The test results demonstrated that only 1 d after the beginning of the hydration, the pozzolanic reaction for silica fume already started, and the reaction exhibited a fast rate in the early stage but a slow one in the later stage. However, the same reaction with a rapidly increasing rate for fly ash just began only 7 d after the beginning of the hydration. In the ternary cementitious system, the hydration degrees for both silica fume and fly ash were reduced with the increasing fly ash content.