Abstract: During the research about crack extension rule at the back-side of adjacent tunnel under the action of blas- ting load adopting the dynamic caustics experimental method, a relationship exists between the crack extension in back-side of tunnel to blasting and arc type stress concentration region which appears with the release of detonation gas. Study shows that after the explosion,the explosion stress wave acts on the crack in the back-side of tunnel firstly, then the quasi-static stress field produced by detonation gas acts on the crack,which plays a main role on the propaga- tion of the crack. Finally,unloading wave acts on crack,and the crack warps. When the arc type stress concentration region moves near the tip of propagation crack,the crack extension velocity reaches to maximum,which equals to the velocity of wave front. Later the relative distance between the crack tip and arc type stress concentration region doesn’t change basically. When the arc type stress concentration region extends cross the crack tip,the extension ve- locity gradually decreases,finally,the crack arrest. Similar changing rule happens on the dynamic stress intensity fac- tor. Dynamic stress intensity factor reaches peak before the arc type stress concentration region runs across the crack tip. Research based on the findings confirms that the detonation gas plays a dominant role to the crack propagation in adjacent tunnel. The researches enable a better understanding on the impact mechanism of explosion load on neighbor- ing tunnel.