Abstract: To investigate the influence of blasting vibration on the stability of small clear distance tunnels excavated through layered rock masses, a transversely isotropic elastic constitutive relationship was first incorporated into the elastic stage of the ubiquitous-joint model, and an improved ubiquitous-joint model was established. Based on the principal stress expressions derived from this model, an improved Zone Safety Index (ZSI_su) was formulated for both the rock matrix and joint planes, enabling the mechanical state of bedded surrounding rock elements to be characterized across the elastic, yielding, and failure stages. Subsequently, a ZSI_su calculation program was developed using the FISH language in FLAC^3D, and its reliability was verified through triaxial compression tests and simplified tunnelling model experiments. Finally, the proposed ZSI_su evaluation index was applied to a blasting construction case of a metro interval tunnelling. The results indicate that, under the original blasting design scheme, blasting loads caused the plastic zones at the arch shoulder and arch waist of the middle rock pillar to become interconnected, and ZSI_su values revealed that this region was in a failure state. After optimizing the blasting scheme based on the spatial distribution characteristics of ZSI_su, both the surrounding rock plastic zones and regions with negative ZSI_su values were effectively eliminated. Moreover, the peak particle velocity was reduced by 43.1% compared with that of the original scheme and was lowered to within the safety threshold.