Abstract：An essential distinction existed between unloading confining pressure and loading rock. Many factors affect on damage mechanism of unloading confining pressure rock. Taking slate in Jinping area as object of study，mechanical property and mesostructure parameter are achieved. The microcrack distribution on engineering rock is reappearance by means of the Monte-Carlo stochastic simulation. Then，the fractal dimension of unloading confining pressure rock under different confining pressures and different stages are obtained. It is a theory and reference for analyzing the microscopic changes of inner medium by means of conventional mechanics method. In conclusion：the fractal dimension of unloading confining pressure rock is magnification with confining pressure. Under the same confining pressure，the fractal dimension of post-peak is larger.
