The effects of carbonation on the microstructure of concrete containing fly ash and expansive admixture are investigated in this paper. Eight mix proportions of the concrete are designed in the experiment with the different replacement levels of the Portland cement with the fly ash and the expansive admixture. The specimens cured in accelerated carbonation condition for 3 days, 7 days and 28 days are used for macro and micro tests. MIP test is used to analyze the influence on internal pore structure of concrete by carbonation reacts. The changes of Ca(OH)2 and CaCO3 contents are measured with TG/DSC analysis. The results indicate that carbonation reaction can make concrete denser. Replacing Portland cement with fly ash and expansive admixture increases the carbonation rate of concrete and fly ash blended in concrete can improve the pore structure of concrete. Apart from the reaction of Ca(OH)2 and CO2, the carbonation reaction of other hydration product also exit in the accelerated carbonation process.