Silane coupling agents (SCAs) are widely used as adhesion promoters to tightly bond glass fiber (GF) and cement matrix (CM) for developing sustainable and high-performance glass fiber-reinforced cementitious (GFRC) composites. This study uses molecular dynamics (MD) simulation to examine the effect of 3-aminopropyltriethoxysilane (APS) on CM/GF interfacial properties. Pure cement and nine cement models bonded to APS-modified GF with APS concentrations of 0.00%, 12.50%, 25.00%, 37.50%, 50.00%, 62.50%, 75.00%, 87.50%, and 100.00% are constructed. It is shown that pure CSH system experiences interfacial delamination. With increasing APS from 0.00% to 37.50%, the failure mode transits from interfacial delamination to CSH delamination, and with increasing APS to 100.00%, it transits to interfacial delamination. Meanwhile, it is measured that with increasing APS from 0.00% to 37.50%, the tensile strength increases from 0.48 to 0.58 GPa and the adhesion energy increases from 0.71 to 0.90 J⋅m−2. Comparatively, with increasing APS to 100.00%, the tensile strength decreases to 0.26 GPa, and the adhesion energy decreases to 0.23 J⋅m−2. Moreover, the interfacial structure and chemical bond analysis demonstrate that with increasing APS from 0.00% to 37.50%, the peak of H2O molecules decreases, which denotes stronger interfacial bonding. With increasing APS from 37.50% to 100.00%, the peak of the H2O molecules increases, which induces low interfacial bonding. These findings provide a basis for optimizing APS concentrations for specific performance enhancements of GFRC materials.