Conventional polyvinyl alcohol (PVA) cementitious composites are susceptible to macroscopic cracking when subjected to reduced compressive strength, diminished toughness, tensile impact and other external loads. In order to address these issues, this study investigates the mechanism of the modifier's influence by modifying PVA fibers with a silane coupling agent (KH560) through experimental studies and molecular dynamics simulations for multiscale computational analyses. Macro test results showed that the mechanical properties of PVA fiber concrete were significantly improved after silane coupling agent modified PVA fiber. The microscopic test results showed that the SCA-modified polymer film acted as a bridging ligament in the interfacial transition zone, effectively prevented crack extension, and enhanced the interfacial adhesion between the organic components of the PVA fibers and the cement matrix. Nanoscale molecular dynamics simulations showed that the hydrated calcium silicate exhibited a stable skeletal structure, while the introduction of SCA expanded the hydrogen bonding interactions at the interface, thus enhancing its stability.