Alkali-activated materials are one of the environmentally friendly materials that are expected to replace cement, but they have disadvantages such as poor resistance to carbonation and there is a lack of unified standards for carbonation research methods for alkali-activated materials. This article proposes a method for testing the carbonation of hardened concrete powder, and studies the carbonation resistance of alkali-activated slag-fly ash mortar (AAM) containing different dosages of plant ash, MgO, Sodium lactate, and C-HT. The experimental results show that when adding plant ash, the carbonation resis-tance of AAM decreases. However, as the dosage of plant ash increases, the carbonation resistance of AAM gradually improves. When the dosage is 6%, the carbonation resistance of AAM is similar to that of the control group. MgO, Sodium lactate, and C-HT can all improve the carbonation resistance of AAM. The effect is most significant when adding 6% MgO or 6% C-HT. Sodium lactate does not signi-ficantly improve the carbonation resistance of AAM. The hardened powder carbonation testing method is not only used to test the carbonation resistance of AAM, but can also be used to accurately compare the carbonation resistance of different additives by measuring the degree and rate of carbonation.