The phenomenon of vibration-induced shear resistance relaxation (ViSRR) refers to the loss of shear resistance in granular materials subjected to vibration, without an observable increase of excess pore pressure, under restricted deforming conditions. Essentially, ViSRR occurs when the total deformation of granular materials is unable to keep up with the plastic deformation induced by the vibration, given the restricted deforming conditions. In this study, ViSRR is investigated through a series of specially designed experiments together with theoretical simulation based on the extended shear transformation zone (STZ) model developed for granular materials. The results demonstrate that ViSRR takes place in granular materials under monotonic shearing when the vibration is superimposed, and the magnitude of shear resistance relaxation depends on the intensity of the vibration. The occurrence of ViSRR is subject to two fundamental prerequisites: superimposed vibration as the driving force for plastic deformation, and restricted total deformation that impedes a granular material's deformability to follow the development of plastic deformation. The simulation results obtained from the extended STZ model are in agreement with laboratory test results, indicating that the model is effective in describing the process of shear resistance relaxation and the accompanying volumetric strain during vibration.