The carbonation behavior of alkali-activated materials (AAMs) over time is crucial for structural durability. In this study, accelerated carbonation tests were conducted on alkali-activated slag-metakaolin (AASM) composite pastes. The effects of various slag content at 0%, 20%, and 40%, and Na2O content at 15%, 17%, 19%, 21%, and 23% on the pore structure, carbonation depth, and material alkalinity of AASM composite pastes were studied. The research results indicated that with the increase in slag content and Na2O content, the pore structure of AASM composite pastes tended to become denser. The influence of slag content and Na2O content on the early carbonation depth was significant. In the 0% slag content group, the carbonation depth at 3d of accelerated carbonation decreased by 61.4% when the Na2O content increased from 15% to 23%. With the increase in slag content, the effect of Na2O content on carbonation depth tended to weaken. In the 40% slag content group, the carbonation depth at 3d of accelerated carbonation remained below 10 mm. As accelerated carbonation progressed, the measured pH value rapidly decreased before 7d of accelerated carbonation and gradually stabilized thereafter. Based on pore structure parameters, predictive models for carbonation depth and material alkalinity of AASM composite pastes were constructed. The results of the accelerated carbonation test were used to validate the model calculations. The results show that the model based on pore structure parameters can effectively predict the carbonation depth and material alkalinity of AASM composite pastes.