In order to explore the influence of fly ash content on the carbonation resistance of cement paste, the mercury intrusion method, evaporative water content method and XRD diffraction pattern were used to study the influence of fly ash content on the internal microstructure of cement paste under carbonation. The results show that the incorporation of fly ash promotes the carbonation of the cement paste specimens, and the carbonation depth of the specimens is the largest when the dosage of fly ash is 30%. With the increase of the fly ash dosage, the total porosity of the cement paste increases, the total porosity after carbonation decreases, the proportion of small pores increases, the pore structure of the cement paste is improved, the calcium alum after carbonation disappears, and calcium carbonate is generated. The carbonation is conducive to the healing of cracks in the cement paste specimens, and the average crack width with 30% fly ash dosage is reduced by 72% compared with that before the maintenance of fly ash.