Abstract: The interaction of a coplanar periodic array of cracks in 3-D space with normal incident S wave, which is concerned in anti-seismic engineering, has been investigated in present paper. We obtained the singular integral equations of the crack openging displacements, and also studied the properties of scattered wave in detail. When crack face is a rectangule, the numerical results of the reflection coefficient and dynamic stress intensity factors around the crack edge have been gotten. The reliability of the results have been checked preliminarily by an application of the balance of rates of energes and extreme state of 2-D space problem.