Abstract: Aiming at the stability of the key blocks in the surrounding rock which near the blasting source in tunnel excavation，the mechanical model of the key block of the stress state is established.According to the balance principle of the rigid body，the contact between the blasting vibration velocity and the physical of block is built to obtain the calculation expression of safety vibration velocity of the key blocks in the tunnel surrounding rock.Analysis shows that the lateral mean normal stress of the key block is proportional to buried depth，the coefficient of rock static lateral pressure have significant influence on the block lateral mean normal stress，and the impact of the ratio of tunnel radius and block height on block lateral mean normal stress is not significant.Combined with practical engineering case，the influences of rock static lateral pressure coefficient，the cumulative damage，the vault and the zone of the maximum shear stress on safety vibration velocity of key block are analyzed，thus providing theoretical basis for the determination of the safety vibration velocity of the tunnel surrounding rock near the blasting source.

Key words: Tunnel engineering, Blasting vibration, Stability of surrounding rock, Safety vibration velocity, Key block