Surrounding rock of tunnel is cut into blocks by structural planes. Tunnel excavation breaks the mechanical balance of the blocks in the natural state. Dynamic loads may promote the slip of a block toward a free surface

which will cause the failure of surrounding rock. However

most applications of block theory only consider the effect of gravity without taking blasting vibration load and seismic load into account. Blasting vibration load and seismic load are transformed into equivalent static loads

and the most unfavorable direction of dynamic load is calculated where the safety factor of block is the lowest. The vector analysis method of block theory is applied to analyze the stability of surrounding rock mass in tunnel considering the effect of dynamic load as well as gravity. The calculated results for Changgang tunnel show that the safety factor of block under dynamic load is obviously lower than that only considering the effect of gravity. Under the action of dynamic load

the original stable block can be turned into a key block and leads to surrounding rock failure

which is consistent with the actual destruction phenomenon recorded in Changgang tunnel.