When tunnels are excavated in rock masses using the drill-and-blast method, determining the extent of blast-induced damage is a significant concern for engineers in terms of safety and cost. This study establishes a quantitative method to calculate the geological strength index and disturbance factor of a rock mass based on P-wave velocity. A case study was conducted to estimate the strength and deformation modulus of a rock mass in an excavation-damaged zone (EDZ) using the modified Hoek–Brown criterion. The results showed that blasting has a considerable weakening effect on the surrounding rock mass, resulting in reductions in compressive and tensile strengths of > 70% and > 90%, respectively. The Riedel–Hiermaier–Thoma (RHT) model in LS-DYNA software was then used to assess the rock mass damage induced by millisecond delay blasting. In this simulation, rock mass parameters estimated by the modified Hoek–Brown criterion were selected to calibrate the RHT model parameters. In addition, an in situ tunnel blasting experiment was used to verify the calibrated parameters of the RHT model. The modeled extent of the EDZ was 1.6 m, and this showed good agreement with field test results. Therefore, this study provides an effective approach to evaluate blast-induced damage during rock mass excavation. Highlights: A rock mass P-wave velocity method is proposed to estimate the geological strength index and disturbance factor. Rock mass parameters and tunnel damage characteristics are estimated using the modified Hoek–Brown failure criterion. Parameters in the RHT model are calibrated using the rock mass parameters estimated by the modified Hoek–Brown criterion. The rationality of the RHT model parameters is verified via a comparison between the modeled result and an in situ experiment of tunnel blasting. [ABSTRACT FROM AUTHOR]