In this paper, a crack is thought to be a physical model in which two semi-infinite uniform media were connected by spring s. The crack is of continuous stress and discontinuous strain. Based on propagation theory of plane wave, it is suggested that coherence in a crack could be indicated by the amplitude ratio or the coupling dynamic stiffness (CDS). The relation between CDS and the ratio has been deduced. The research is used to check the effect of grouting crevices in the Dunhuang Caves.