<p>This paper was focused on the drying shrinkage mechanism and the development of the corresponding prediction model for ambient-cured binary FA-GBFS geopolymer mortar (GPM). GPMs were tested with varying SiO₂/Na₂O molar ratio (Ms) values (0.65, 0.85, 1.0, and 1.16), Na₂O contents (5, 6, 7, and 8%) and macro basalt fiber volume contents (0.5, 1, 2, and 3%). The compressive strength, flexural strength, and drying shrinkage of the GPMs were determined. The observed drying shrinkage behavior was correlated with Low-Field Nuclear Magnetic Resonance (LF-NMR) and Scanning Electron Microscopy (SEM). This paper also analyzed the interaction among calcium oxide (CaO) content, water–binder ratio (W/B), Ms and Na₂O content on drying shrinkage according to existing research. The results indicate a positive correlation between drying shrinkage and both Ms and Na₂O contents, while macro basalt fiber was found to inhibit drying shrinkage. The increased drying shrinkage of GPM was attributed to the formation of C–(A)–S–H gels, which reduced porosity and increased mesopores. Notably, the drying shrinkage mechanism of the FA-GBFS-based GPM in this study exhibits similarities to that of GBFS-based GPM. The developed multivariate nonlinear regression model, incorporating CaO content, W/B ratio, Ms, and Na₂O content, achieved a level of precision comparable to the experimental data and existing research.</p>