Making full use of fly ash (FA) is an important part of sustainable materials development in the word. This paper discussed the carbonation resistance of concrete with 10%, 20% and 25% fly ash replacement ratios(FA%). The process was accelerated in artificial carbonation laboratory by controlling 20% concentration of CO2, 10°C~30°C temperature and 50%~70% relative humidity(RH). The depth of carbonation could be observed by spraying phenolphthalein solution on the fresh broken concrete surface. The test results show that adding fly ash into concrete results in the decrease of cement consumption and calcium hydroxide, so carbonation rate increases. But with the increase of FA% and carbonization time, the drop speed of carbonation rate slower, the differences in the microstructure are detailed. In addition, the carbonation rate increases when the ambient temperature varies from 10°C to 30°C and ambient RH falls from 70% to 50%. It concludes that the 10°C ambient temperature and 70% ambient RH are superior to other environmental conditions for enhanceing concrete carbonation resistance, and mixing 10%~25% FA into concrete have little influence on carbonation resistance of concrete if have adequate curing time.