Abstract: Based on a fractal cellular automata model,the mechanism of self-organized criticality(SOC) of granular mixtures was investigated.The present sandpile experiments with an one-side slope show that the self-organized criticality of granular mixtures is strongly influenced by the non-uniform degree of granular materials.The fractal dimension of grading was calculated using the fractal theory. The results show that there exists a fractal feature in the grading of sandpiles with SOC.So it was supposed that if the fabric characteristics of a system has a fractal nature,a large-dimension non-uniform sandpile,as such a system,presents SOC.A cellular automata model describing non-uniform coefficient and fractal fabric characteristic was built to explain the assumption.Through numerical simulations it was found that a sandpile model takes on SOC when cellular arrangement submits to a fractal distribution to testify the assumption.A discussion about the criterion of SOC shows that the fractal fabric characteristics of a system is the necessary condition for SOC of large-dimension granular mixtures.