The present study endeavored to elucidate the effect of molar ratio of calcium sulfate over ye'elimite (m-value) on the reaction of calcium sulfoaluminate (CSA) cement/blast furnace slag blends under an accelerated carbonation condition. CSA cement was replaced with slag at a level of 50%. Paste samples were exposed to the accelerated carbonation condition for 28 days after a sufficient precuring period. The phase evolution, local Al and Si frameworks, physical properties, and CO2 uptake aspects of the samples were explored. The test results indicated that the phase composition of the samples was not altered regardless of the m-value. Meanwhile, an increase in the m-value was revealed to enhance the reaction degree of Si in the slag to form C-(A)-S-H after carbonation. The CO2 uptake level of the sample with the highest m-value (2.0) was calculated to be the lowest, signifying that the pore structures and strength development upon carbonation were influenced more by the occurrence of C-(A)-S-H formation rather than by the packing effect of calcium carbonates.