Abstract: To achieve low-carbon and high-performance building materials, a multi-scale fiber-reinforced geopolymer composite (FRGC) was prepared using industrial solid wastes. The workability, mechanical properties, and carbon emission characteristics of the material were systematically investigated. An orthogonal experimental design was employed to evaluate the effects of slag–fly ash ratio, alkali activator dosage, PVA fiber length, and silica fume content on the material performance, and the optimal mix proportion was determined. On this basis, physical and mechanical tests were conducted by varying the PVA fiber volume fraction (0–1.5%) and calcium carbonate whisker content (0–2.0%). The results indicate that appropriate incorporation of PVA fibers and calcium carbonate whiskers can achieve synergistic reinforcement through bridging, nucleation, and filling effects, with the flexural and compressive strengths increasing by up to 31% and 41%, respectively. However, excessive dosages lead to agglomeration, which weakens the reinforcing effect. Based on the experimental results, prediction models for the flexural and compressive strengths of FRGC were established and verified. SEM-EDS analysis reveals a dense internal microstructure dominated by the formation of C-(A)-S-H and N-A-S-H gels, with PVA fibers and whiskers forming a multi-scale reinforcement mechanism. Carbon emission assessment shows that the carbon emissions of FRGC are reduced by approximately 50% compared with ordinary Portland cement mortar, with a minimum environmental efficiency factor of 0.34, demonstrating significant carbon reduction benefits while improving mechanical performance.