The present study investigated the carbonation of calcium sulfoaluminate (CSA) cement blended with blast furnace slag. Paste samples were prepared by substituting CSA cement with slag at dosages of 0, 30, 50, and 70 wt%. The samples were carbonated for 28 days at a CO2 concentration of 3% after a sufficient pre-curing period. The test results indicated that slag incorporation did not affect the phase assemblage of the sample regardless of carbonation, while the incorporated slag promoted the formation of monosulfate and stratlingite upon carbonation in which this observation is closely related to the improved reaction degree of Si in the slag by carbonation. Meanwhile, monosulfate and ettringite were the first phases to be carbonated, while the C-A-S-H gel formed due to the carbonation of belite, thereby enhancing the compressive strength by refining the pore size in the gel pore region. On the other hand, the carbonation degree of the samples vastly increased as the slag content increased due to a small amount of ettringite formed along with the low reaction degree of slag which induced the development of macropores.