Semi-flexible pavement (SFP) is a composite material comprising porous asphalt and cementitious grout. Its asphalt-grout interfacial transition zones (ITZ) are prone to cracking. While interfacial modifiers can improve ITZ crack resistance, their effects on microscopic morphology and chemical composition remain unclear, limiting the understanding of mechanisms influencing ITZ performance and the optimised use of modifiers. To achieve this challenge, the study characterised the microscopic morphology and chemical composition of the asphalt-grout ITZ in SFP, particularly those modified by immersion in a silane coupling agent solution. The asphalt-grout ITZ in SFP material is analyzed using a series of micromechanics techniques, including scanning electron microscopy (SEM) for microscopic morphology, energy dispersive spectrum analysis (EDS) for chemical components, X-ray diffraction (XRD) for phase composition, and Fourier-transform infrared spectroscopy (FTIR) for chemical bond information. The results indicate that an interfacial modifier can effectively enhance the microscopic bonding between asphalt and grout material, reducing the likelihood of tiny cracks between these two phases. The asphalt-grout ITZ exhibits a distinct double-layer structure, with altered morphology compared to the bulk material, including hexagonal calcium hydroxide crystals aggregation, which is more prone to cracking than hydrated calcium silicate (C–S-H) gel. Element distribution at the enhanced interface is more continuous, with synchronous enrichment. These findings provide insights into proactive strategies for improving crack resistance at asphalt-grout ITZ, thereby strengthening the road performance of SFP materials.