Abstract: The effects of rock characteristics on the propagation of explosion stress wave are studied and the blast action and the crack effects in the orthotropic rock are analysed through fractal principle. The gaps in rock are considered as a kind of damage and the relationship between the porosity of rock and attenuation index of stress wave with distances is deduced from the fractal dimensions. As an important parameter, the porosity directly affects the deformation and mechanical characteristics of rock. This parameter, introduced into the attenuation index of explosion stress wave with distances, embodies the role of rock characteristics in the blast load. This coincides with the experience and experiment results. The paper compares the attenuation indexes in two perpendicular directions in the orthotropic rock . The quantitative relationship between spacing of blast holes in two perpendicular directions are given, which provides the theoretical basis for blast engineering design.