Fifteen groups of real sand-pile model are introduced,whose self-organized criticality in granular mixtures is strongly influenced by non-uniform degree of granular material.Three different dynamic behaviors showed in the experiment,such as quasi-period distribution,normal distribution and power law distribution,are analyzed.It is deduced that non-uniform degree is neither used to describe the self-comparability of system configuration nor used as the criterion of self-organized criticality(SOC).Fractal theory is applied to study fractal characteristics of sand-pile structure.It is showed that sand pile takes on a good fractal structure as non-uniform degree is between 1.53 and 5.0. Fractal dimension rises fast when non-uniform degree is less than 2.85,and rises slow when it is greater than 2.85 but does not exceed 3.0.Moreover,through the analysis of the intrinsic relation between fractal characteristics of sandpile and SOC,the system presented SOC is found to have a steady fractal structure.The criterion for SOC is discussed and application of the SOC theory is prospected.Application of SOC theory in the forecast and prediction of mountain hazards and their control engineering design are discussed at length.