Alkali-activated fly ash (AAF) is a promising material that exhibits comparable material properties as cement-based materials but with much less CO2 emission. In the present work, the effect of activating solution (SiO2 and Na2O content) on the performance of AAF was studied by means of isothermal calorimetry and X-ray diffraction analysis. Meanwhile, the pore structure of AAF was examined by mercury intrusion porosimetry combined with environmental scanning electron microscope. The results indicate that increasing the sodium oxide content leads to a higher extent of reaction, denser matrix and higher possibility of crystallization, corresponding to a higher compressive strength of AAF. The addition of silica in the alkaline solution retards the reaction rate and zeolite formation, while improves the microstructure of the matrix. Therefore, there is an optimal value for SiO2 with respect to the Na2O content for the AAF in this study.