Alkali-activated slag cement is a kind of low carbon cementitious material, and the study of its carbonization resistance has become one of the hotspots. In this paper, the effects of saturation degree (90%, 70% and 50%) on the carbonization resistance (compressive strength retention rate and carbonization depth etc.) of alkali-activated slag mortar (ASm) after carbonization were studied by using water glass (WG) as the activator. The carbonation products and pore structure of ASm were studied by XRD, TGA-DTG, NMR and MIP, and the influence mechanism of saturation degree on the carbonation resistance of ASm was revealed. Studies have shown that, with the decrease of saturation degree, CO2 enters the ASm more easily, the decalcification of C-(A-)S-H gel is more serious, the amonut of highly crosslinked C-(A-)S-H gel increases, its polymerization degree increases, the mean chain length becomes longer, the total porosity and average pore size increase, and the amount of macropores increases, and diffusible pore volume increases. The carbonation product CaCO3 is mostly unstable aragonite and vaterite, and the carbonization resistance of ASm deteriorates. When the saturation degree drops from 90% to 70%, the compressive strength and compressive strength retention rate of ASm significantly decrease, and carbonization depth significantly increases; when the saturation degree drops from 70% to 50%, the compressive strength and compressive strength retention rate of ASm decrease slightly, and carbonization depth increases slightly.