To study the gap structure and its distribution in the rock
a statistical self-similarity based fractal model describing gap structure was established.The calculate formula of the dimension of self-similarity about gap distribution was obtained through mathematical expectation of random fragmentation number and selfsimilar ratio.The relationship between the fractal dimension of initial structure and fractal dimension of crushed structural was discussed.The above work laid theoretical foundation for further study of the gap structure of rock and its cracking mechanism.