Abstract: Effect of the initial geo-stress unloading on the circular tunnel boundary is studied,and the damage area is calculated by use of combined method of theoretical analysis,numerical simulation by Dynamic FEM and verification with field monitoring vibration data.The results indicate that the damage area induced by the dynamic unloading is larger than that induced by the static unloading during blasting excavation.The main factors,which affect the dynamic stress of unloading,are side-face stress coefficient λ and angle θ.The larger the λ is,the more remarkable the unloading effect is,and the damage area is much bigger under given unloading rate.The results of theoretical analysis are validated by the monitoring damage data.