Mullite as one of the major aluminum-containing minerals in pulverized coal fly ash (PCFA) has a stable crystal structure, resulting in the difficulty in aluminum extraction from PCFA. Na2CO3-assisted thermal activation of PCFA is an efficient approach to activate the reactivity of mullite in PCFA. A clear understanding of the decomposition and transformation mechanism of mullite calcined with Na2CO3 is of great significance to clarify the activation mechanism of PCFA and the further process optimization. In this work, the thermal decomposition behavior, the phase transformation and the microstructure change were investigated in detail using thermo-gravimetric and differential scanning calorimetric (TG-DSC), X-ray diffraction (XRD) and magic angle spinning nuclear magnetic resonance (MAS-NMR). The results showed that the phase transformation process was mullite firstly converting into sodium aluminum silicate (SAS, Na1+xAl1+xSi1-xO4, 0 < x < 1) at< 800 °C and then to other minerals with higher structure stability, such as sodium aluminate and sodium silicate, with the increasing of Na2CO3 amount at> 800 °C. The reaction mechanism could be interpreted in terms of the combination change of different oxygen coordination. During the calcination, the Na2O decomposed from Na2CO3 would enter in mullite through filling oxygen vacancies and then react with the oxygen atoms around the AlVI, resulting in the formation of the AlO4 in SAS. Further increase in the calcination temperature and the Na2CO3 amount, the transforming from mullite to SAS would be obviously enhanced. When the temperature was> 800 °C, the residual AlO6 originated from mullite was further transformed into AlO4 with the larger amount of Na2CO3. Meanwhile, the bridging oxygen bonds between AlO4 and SiO4 in SAS were broken by the attacking of Na2O. Subsequently, all the obtained AlO4 and SiO4 as well as Na+ ion could be further recombined in a specific manner to transform into sodium aluminates (AlO4-AlO4), sodium silicates (SiO4-SiO4) and sodium aluminosilicates (AlO4-SiO4).