Low-carbon and highly printable cementitious materials are crucial for the practical application of extrusion-based three-dimensional concrete printing (3DCP). This study develops and optimizes a one-part alkali-activated concrete suitable for 3D printing through an integrated experimental and evaluation approach. An orthogonal experimental design was employed to investigate the effects of precursor ratio (ground granulated blast-furnace slag, GGBFS, to fly ash, FA), water-to-binder ratio, activator dosage, and retarder content on fresh-state properties, rheological behavior, setting characteristics, and mechanical performance. The optimal mixture was determined using the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) multi-criteria decision method. The mixtures exhibited suitable rheology, with a yield stress of 90–141 Pa and a plastic viscosity of 3.5–7.2 Pa·s, a setting time of 40–96 min, and mechanical performance with compressive and flexural strengths of 29–71 MPa and 4.2–6.9 MPa, respectively. The optimal mixture provided a 95-min printing open time and an acceptable pumping pressure of 1.77 MPa, while full-scale tests confirmed stable extrusion and good print quality. Furthermore, within the defined cradle-to-gate, materials-stage boundary and the adopted inventory factors, the optimized alkali-activated mixture exhibited an embodied CO₂ emission of 0.113 kg CO₂/L, which is approximately 61% lower than that of the reference cement-based printable mixture. The proposed approach provides a systematic framework for designing low-carbon, high-performance one-part alkali-activated materials for extrusion-based 3D concrete printing applications.