Abstract: Based on the characteristic of blasting construction, the dynamic effect caused by blasting construction for the typical double side excavation method is studied by numerical simulation. The general laws of dynamic effect are proposed at the boundary of the existing tunnel. The secondary supporting time is important to the existing tunnel in the region influenced by blasting construction, and the key locations which blasting construction influences the stability of existing tunnel are given. The analysis shows that the dynamic effects caused by blasting on the stability of surrounding rock of existing tunnel are mainly the tensile damage at the surface of surrounding rock caused by blasting stress wave. The most serious damage locates at the face-blast-wall, arch feet, arch roof of the existing tunnel. The results provide some reference for the blasting construction of the tunnels with small spacing.