Cementitious materials in early-age carbonation curing with varying sand-to-cement ratios， water-to-cement ratios， and specimen sizes were investigated by AC impedance technique. An optimized equivalent circuit was established， and corresponding electrical parameters were obtained to analyze the changes in the impedance spectrum of cement-based materials in early-age carbonation curing. The results show that the mass increase ratio of specimens increases with the water-to-cement ratio and decreases with the sand-to-cement ratio. The average mass increase ratio of the neat slurry group increase is 11.29%， which is 2 to 3 times higher than that of the mortar group. The larger size of specimens results in the higher mass increase ratio. In carbonation curing conditions， the diameter of the arc in the high-frequency area of AC impedance spectrum increases， while the slope of the straight line in the low-frequency area decreases in cementitious materials. A relationship model between the mass increase ratio and AC impedance spectrum parameters is established. The charge transfer resistance is used to predict the CO₂ absorption of the cementitious materials in carbonation curing. AC impedance spectrum of cementitious materials in carbonation early-age curing contributes to the development of corresponding non-destructive monitoring technologies.