In the experiments of real-time holographic interferometry, some dark shadow areas appear among the interference fringes. The dark shadow areas play a very important role in the fracture process. When the applied force increases near to the fracture stress, these dark areas extend in an accelerative manner and quickly leads to total rupture of the specimen. In fact, the appearing of these dark shadow areas are the phenomena of caustic in geometry optics. The shadow area is so-called shadow spot and the factor of stress intensity can be calculated by measuring the size of corresponding length of the shadow spot according to fracture mechanics theory. The research works on the relationship between interference fringes and the factor of stress intensity are introduced. The quantitative expression of this relationship has been found and introduced. It provides a basic foundation for calculating the results of the experiment in real-time holographic interferometry combine with caustic method.