Abstract: The fracture mechanics solusion of the stress field near the tip of a closed crack under compression and shear is given and a new method for prefabricating closed cracks is developed in this paper. The compression-shear closed crack is studied by means of photoelasticlty,and the changes of both the stress field near the crack tip and the distribution of friction on the closed crack are studied. Futhermore,the interaction of the singular stress field between,two initially closed collinear cracks is also studied by photoelasticity. The stress intensity factor (SIF) of the compression-shear closed crack is computed by the finite element nethod. It shows that the results in the paper are correct. Finally,some applications in the research on the seismic focus are discussed.