Abstract: The method of dynamic caustics is adopted to investigate the mechanism of original crack existing in the surrounding rock of adjacent tunnel under the action of blasting load.Conclusions are drawn as follows.Under the action of blasting load,dynamic fraction toughness of pre-crack is not related to the dip angle of pre-crack and the free surface of tunnel.However,the existence of free surface makes pre-crack initiation in advance.In general,the propagation path of pre-crack facing the blasting side of adjacent tunnel under the action of blasting load is related to the propagation path of principal crack from the blast-hole,the free surface of tunnel as well as its section form.Initial propagation path presents approximate vertical upward because of the action of tension wave reflected by the free surface of tunnel.The middle and later periods of pre-crack propagation,the propagation path of principal crack and the stress distribution characteristics of free surface have great effect on the propagation path of pre-crack.The stress distribution characteristics of surrounding rock facing the blasting side are analyzed through the LS-DYNA.From the base angle of side wall to vault facing the blasting side of tunnel,effective stress presents the variation rule of decreasing-increasing-decreasing,of which the maximum points lie to the base angle and the location between sprung arch and vault,the minimum point lie to the side wall.As a result,the pre-crack propagation path is partial to the arch parts and arrest at certain location between the sprung arch and vault.